Added option to force Heap Buffer for headers

[denyska]

Rationale.

• A good number of native memory leaks are coming from the library user code, i.e.

            request = new ThriftRequest.Builder<KeyValue.setValue_args>("keyvalue-service", "KeyValue::setValue")
                    .setTimeout(1000)
                    .setBody(setValue)
                    .build();
            // do something else that results in the error
            subChannel.send(request);

Meaning if there is an error between ThriftRequest / ThriftResponse constructed and it handed over to tchannel, then Memory will leak. Instead of fixing all of usages, we can take small performance hit (see next bullet).

• Body is majority of the Tchannel payload and com.uber.tchannel.messages.ThriftSerializer#encodeBody uses heap ByteBuf, using Heap ByteBuf for the headers won't make big difference.

[codecov[bot]]

Codecov Report

Merging #211 into master will decrease coverage by 0.19%. The diff coverage is 62.5%.

@@             Coverage Diff              @@
##             master     #211      +/-   ##
============================================
- Coverage     73.56%   73.37%   -0.19%     
  Complexity       10       10              
============================================
  Files            87       87              
  Lines          2753     2760       +7     
  Branches        345      348       +3     
============================================
  Hits           2025     2025              
- Misses          506      509       +3     
- Partials        222      226       +4

Impacted Files	Coverage Δ	Complexity Δ
...a/com/uber/tchannel/messages/ThriftSerializer.java	76.47% <62.5%> (-5.01%)	0 <0> (ø)
...java/com/uber/tchannel/handlers/RequestRouter.java	72.48% <0%> (-3.67%)	0% <0%> (ø)
...ava/com/uber/tchannel/handlers/ResponseRouter.java	80.23% <0%> (ø)	0% <0%> (ø)	:arrow_down:

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