[aggregator] Reduce error handling overhead in rawtcp server

[vdarulis]

What this PR does / why we need it:

The errors escape to the heap for no good reason, and makes rate-limiting more expensive than it should be:

src/aggregator/server/rawtcp/server.go:146:27: addUntimedError literal escapes to heap
src/aggregator/server/rawtcp/server.go:150:27: addUntimedError literal escapes to heap
src/aggregator/server/rawtcp/server.go:154:27: addUntimedError literal escapes to heap
src/aggregator/server/rawtcp/server.go:158:29: addForwardedError literal escapes to heap
src/aggregator/server/rawtcp/server.go:162:25: addTimedError literal escapes to heap
src/aggregator/server/rawtcp/server.go:166:25: addTimedError literal escapes to heap
src/aggregator/server/rawtcp/server.go:170:31: addPassthroughError literal escapes to heap

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 #3183 (0fac338) into master (f6450c1) will increase coverage by 0.0%.
The diff coverage is 85.7%.

@@            Coverage Diff            @@
##           master    #3183     +/-   ##
=========================================
  Coverage    72.2%    72.3%             
=========================================
  Files        1087     1086      -1     
  Lines      101840   100417   -1423     
=========================================
- Hits        73592    72617    -975     
+ Misses      23161    22752    -409     
+ Partials     5087     5048     -39     

Flag	Coverage Δ
aggregator	75.8% <85.7%> (+0.1%)	⬆️
cluster	85.0% <ø> (ø)
collector	84.3% <ø> (ø)
dbnode	78.7% <ø> (+0.3%)	⬆️
m3em	74.4% <ø> (ø)
m3ninx	73.1% <ø> (-0.2%)	⬇️
metrics	20.0% <ø> (ø)
msg	74.2% <ø> (-0.1%)	⬇️
query	67.3% <ø> (ø)
x	80.2% <ø> (-0.5%)	⬇️

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