Encoding buffer resizing

[camdencheek]

Description of Changes

Fixes the short destination buffer error introduced by the multiple encoding support by doubling its capacity each time we hit that error.

Additionally, made a bunch of incremental refactors:

• Pulled out duplicated code into the emit closure
• Moved the read to end logic out of the defer statement for proper error handling and more linear logic flow
• Simplified channel operations and goroutine wait logic, fixing a bug with unclean Stop() in the process (which should help stabilize our tests a bit)
• Pulled the nasty scanner stuff into its own object, which isolates the confusing parts

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[codecov]

Codecov Report

Merging #58 into master will decrease coverage by 0.96%.
The diff coverage is 79.72%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
- Coverage   75.94%   74.98%   -0.96%     
==========================================
  Files          61       63       +2     
  Lines        3691     3833     +142     
==========================================
+ Hits         2803     2874      +71     
- Misses        665      716      +51     
- Partials      223      243      +20     

Impacted Files	Coverage Δ
operator/buffer/memory_buffer.go	91.53% <ø> (ø)
operator/builtin/output/elastic.go	17.35% <0.00%> (ø)
operator/duration.go	76.47% <ø> (ø)
operator/helper/expr_string.go	87.88% <ø> (ø)
operator/helper/namespace.go	100.00% <ø> (ø)
operator/helper/severity.go	96.97% <ø> (ø)
operator/builtin/output/google_cloud.go	43.58% <27.03%> (ø)
entry/nil_field.go	40.00% <40.00%> (ø)
operator/builtin/input/udp.go	77.36% <44.44%> (ø)
operator/builtin/transformer/restructure.go	73.81% <50.00%> (ø)
... and 55 more

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[djaglowski]

Log Files	Logs / Second	CPU Average (%)	Memory Average (MB)
1	1000	2.9139338	30.354795
1	5000	6.2760353	33.39359
1	10000	11.362376	44.68494
1	50000	40.432327	99.75296
1	100000	70.62374	189.81236
10	100	5.0001464	27.50633
10	500	8.258801	33.395878
10	1000	12.552064	45.369072
10	5000	45.82837	92.670525
10	10000	81.502075	187.86732