[aggregator] Pack some tcp client structs for lower memory utilization

[vdarulis]

What this PR does / why we need it:

Reorder some fields of per-connection/per-queue structs:

/Users/vytenis/gocode/m3/src/metrics/encoding/protobuf/buffer.go:30:13: struct with 40 pointer bytes could be 24
/Users/vytenis/gocode/m3/src/aggregator/client/conn_options.go:117:24: struct with 120 pointer bytes could be 64
/Users/vytenis/gocode/m3/src/aggregator/client/payload.go:67:19: struct with 416 pointer bytes could be 408
/Users/vytenis/gocode/m3/src/aggregator/client/ref_count.go:27:15: struct with 16 pointer bytes could be 8
/Users/vytenis/gocode/m3/src/aggregator/client/writer.go:63:13: struct with 168 pointer bytes could be 120
/Users/vytenis/gocode/m3/src/aggregator/client/writer.go:536:20: struct with 24 pointer bytes could be 16

Special notes for your reviewer:

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

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

[codecov]

Codecov Report

Merging #3037 (bcf6fc9) into master (bcf6fc9) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3037   +/-   ##
======================================
  Coverage    72.3%   72.3%           
======================================
  Files        1080    1080           
  Lines       99552   99552           
======================================
  Hits        72036   72036           
  Misses      22520   22520           
  Partials     4996    4996           

Flag	Coverage Δ
aggregator	75.8% <0.0%> (ø)
cluster	84.8% <0.0%> (ø)
collector	84.3% <0.0%> (ø)
dbnode	78.9% <0.0%> (ø)
m3em	74.4% <0.0%> (ø)
m3ninx	73.1% <0.0%> (ø)
metrics	19.8% <0.0%> (ø)
msg	74.2% <0.0%> (ø)
query	67.3% <0.0%> (ø)
x	80.2% <0.0%> (ø)

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