Check the level before writing debug messages.

[hackerwins]

What this PR does / why we need it:

Check the level before writing debug messages.

Performance was dropping to generate unnecessary debug messages. To avoid this, check the level before outputting a debug message.

goos: darwin
goarch: amd64
pkg: github.com/yorkie-team/yorkie/test/bench
cpu: Intel(R) Core(TM) i5-4690 CPU @ 3.50GHz
BenchmarkTextEditing
    text_editing_bench_test.go:59: processing... 10000 174.504879ms
    text_editing_bench_test.go:59: processing... 20000 109.470337ms
    text_editing_bench_test.go:59: processing... 30000 4.806405038s
    text_editing_bench_test.go:59: processing... 40000 216.480363ms
    text_editing_bench_test.go:59: processing... 50000 112.252058ms
    text_editing_bench_test.go:59: processing... 60000 808.748392ms
    text_editing_bench_test.go:59: processing... 70000 667.073535ms
    text_editing_bench_test.go:59: processing... 80000 9.17087822s
    text_editing_bench_test.go:59: processing... 90000 2.180935603s
    text_editing_bench_test.go:59: processing... 100000 1.731117444s
    text_editing_bench_test.go:59: processing... 110000 1.056575369s
    text_editing_bench_test.go:59: processing... 120000 124.693308ms
    text_editing_bench_test.go:59: processing... 130000 513.933027ms
    text_editing_bench_test.go:59: processing... 140000 54.806386806s
    text_editing_bench_test.go:59: processing... 150000 1m54.853097078s
    text_editing_bench_test.go:59: processing... 160000 1.055604723s
    text_editing_bench_test.go:59: processing... 170000 941.902282ms
    text_editing_bench_test.go:59: processing... 180000 15.575431116s
    text_editing_bench_test.go:59: processing... 190000 1.674996829s
    text_editing_bench_test.go:59: processing... 200000 15.967675598s
    text_editing_bench_test.go:59: processing... 210000 224.987399ms
    text_editing_bench_test.go:59: processing... 220000 154.559864ms
    text_editing_bench_test.go:59: processing... 230000 155.082594ms
    text_editing_bench_test.go:59: processing... 240000 1.347651533s
    text_editing_bench_test.go:59: processing... 250000 4.099265955s
BenchmarkTextEditing-4   	       1	240883800421 ns/op

Which issue(s) this PR fixes:

Address #251

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #253 (8ff9d60) into main (23677db) will decrease coverage by 1.54%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #253      +/-   ##
==========================================
- Coverage   51.41%   49.87%   -1.55%     
==========================================
  Files          54       54              
  Lines        4403     4417      +14     
==========================================
- Hits         2264     2203      -61     
- Misses       1932     1997      +65     
- Partials      207      217      +10     

Impacted Files	Coverage Δ
pkg/document/document.go	51.47% <0.00%> (-0.77%)	⬇️
pkg/document/internal_document.go	21.66% <0.00%> (-0.37%)	⬇️
pkg/document/json/rich_text.go	51.02% <0.00%> (-13.20%)	⬇️
pkg/document/json/text.go	42.46% <0.00%> (-9.65%)	⬇️
yorkie/backend/sync/memory/pubsub.go	48.35% <0.00%> (-37.37%)	⬇️
pkg/document/json/rga_tree_split.go	73.42% <0.00%> (-7.21%)	⬇️

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[zeikar]

LGTM 👍