Optimize write buffer capacity

[caspervonb]

Default capacity is 8k, this bumps it to 64k and makes it configurable.

[caspervonb]

Not completely trusting the benchmark in this but might be something in bumping the buffer size a bit more.

async-nats: publish throughput/32                                                                           
                        time:   [51.839 µs 52.507 µs 53.419 µs]
                        thrpt:  [57.129 MiB/s 58.121 MiB/s 58.870 MiB/s]
                 change:
                        time:   [-2.2753% -0.1841% +1.9606%] (p = 0.88 > 0.05)
                        thrpt:  [-1.9229% +0.1845% +2.3283%]
                        No change in performance detected.
Found 1 outliers among 30 measurements (3.33%)
  1 (3.33%) high mild
async-nats: publish throughput/1024                                                                           
                        time:   [75.575 µs 76.391 µs 77.555 µs]
                        thrpt:  [1.2297 GiB/s 1.2484 GiB/s 1.2619 GiB/s]
                 change:
                        time:   [-42.047% -40.781% -39.589%] (p = 0.00 < 0.05)
                        thrpt:  [+65.532% +68.864% +72.554%]
                        Performance has improved.
Found 3 outliers among 30 measurements (10.00%)
  3 (10.00%) high mild
async-nats: publish throughput/8192                                                                           
                        time:   [193.45 µs 201.42 µs 208.77 µs]
                        thrpt:  [3.6545 GiB/s 3.7877 GiB/s 3.9438 GiB/s]
                 change:
                        time:   [-77.907% -77.243% -76.602%] (p = 0.00 < 0.05)
                        thrpt:  [+327.39% +339.43% +352.64%]
                        Performance has improved.
Found 2 outliers among 30 measurements (6.67%)
  2 (6.67%) high mild

async-nats: publish messages amount/32                                                                           
                        time:   [50.903 µs 51.252 µs 51.622 µs]
                        thrpt:  [1.9371 Melem/s 1.9512 Melem/s 1.9645 Melem/s]
                 change:
                        time:   [-3.5160% -2.2907% -1.0012%] (p = 0.00 < 0.05)
                        thrpt:  [+1.0114% +2.3444% +3.6442%]
                        Performance has improved.
Found 1 outliers among 30 measurements (3.33%)
  1 (3.33%) high mild
async-nats: publish messages amount/1024                                                                           
                        time:   [75.895 µs 76.682 µs 77.578 µs]
                        thrpt:  [1.2890 Melem/s 1.3041 Melem/s 1.3176 Melem/s]
                 change:
                        time:   [-45.543% -44.339% -43.167%] (p = 0.00 < 0.05)
                        thrpt:  [+75.953% +79.659% +83.631%]
                        Performance has improved.
async-nats: publish messages amount/8192                                                                           
                        time:   [199.41 µs 202.28 µs 205.71 µs]
                        thrpt:  [486.13 Kelem/s 494.36 Kelem/s 501.49 Kelem/s]
                 change:
                        time:   [-77.858% -77.177% -76.493%] (p = 0.00 < 0.05)
                        thrpt:  [+325.41% +338.15% +351.64%]
                        Performance has improved.

subscribe amount/32     time:   [1.2371 ms 1.2466 ms 1.2570 ms]                                
                        thrpt:  [79.555 Kelem/s 80.219 Kelem/s 80.832 Kelem/s]
                 change:
                        time:   [+258.28% +262.69% +267.42%] (p = 0.00 < 0.05)
                        thrpt:  [-72.783% -72.428% -72.089%]
                        Performance has regressed.
Found 1 outliers among 30 measurements (3.33%)
  1 (3.33%) low mild
subscribe amount/1024   time:   [319.44 µs 322.63 µs 325.78 µs]                                 
                        thrpt:  [306.95 Kelem/s 309.96 Kelem/s 313.04 Kelem/s]
                 change:
                        time:   [-20.470% -18.955% -17.315%] (p = 0.00 < 0.05)
                        thrpt:  [+20.941% +23.388% +25.738%]
                        Performance has improved.
Found 1 outliers among 30 measurements (3.33%)
  1 (3.33%) high mild
subscribe amount/8192   time:   [1.3011 ms 1.7104 ms 2.1781 ms]                                  
                        thrpt:  [45.911 Kelem/s 58.464 Kelem/s 76.860 Kelem/s]
                 change:
                        time:   [-53.005% -38.319% -21.793%] (p = 0.00 < 0.05)
                        thrpt:  [+27.866% +62.126% +112.79%]
                        Performance has improved.