[coordinator] Ensure does not grow M3Msg buffer if message over max size

[robskillington]

What this PR does / why we need it:

On the occasion that a message is sent over max size, it can cause large allocations. This avoids increasing the buffer before checking if over max size to avoid these large allocations when this occurs.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[arnikola]

LGTM

[codecov]

Codecov Report

Merging #2207 into master will decrease coverage by 0.3%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master   #2207     +/-   ##
========================================
- Coverage    72.1%   71.7%   -0.4%     
========================================
  Files        1022    1022             
  Lines       88793   88793             
========================================
- Hits        64041   63727    -314     
- Misses      20441   20746    +305     
- Partials     4311    4320      +9     

Flag	Coverage Δ
#aggregator	82.0% <ø> (-0.1%)	⬇️
#cluster	85.2% <ø> (-0.2%)	⬇️
#collector	82.8% <ø> (+11.4%)	⬆️
#dbnode	78.0% <ø> (-0.9%)	⬇️
#m3em	74.4% <ø> (ø)
#m3ninx	72.3% <ø> (ø)
#m3nsch	51.1% <ø> (ø)
#metrics	17.6% <ø> (ø)
#msg	74.0% <100.0%> (-0.8%)	⬇️
#query	68.4% <ø> (ø)
#x	83.2% <ø> (-0.3%)	⬇️

---

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[codecov]

Codecov Report

Merging #2207 into master will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master   #2207     +/-   ##
========================================
+ Coverage    72.1%   72.2%   +0.1%     
========================================
  Files        1022    1022             
  Lines       88793   88793             
========================================
+ Hits        64041   64132     +91     
+ Misses      20441   20358     -83     
+ Partials     4311    4303      -8     

Flag	Coverage Δ
#aggregator	82.0% <ø> (-0.1%)	⬇️
#cluster	85.2% <ø> (-0.2%)	⬇️
#collector	82.8% <ø> (+11.4%)	⬆️
#dbnode	79.0% <ø> (+0.2%)	⬆️
#m3em	74.4% <ø> (ø)
#m3ninx	72.3% <ø> (ø)
#m3nsch	51.1% <ø> (ø)
#metrics	17.6% <ø> (ø)
#msg	74.8% <100.0%> (ø)
#query	68.4% <ø> (ø)
#x	83.2% <ø> (-0.3%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 07e7df8...4e34743. Read the comment docs.