Refactor more code to adhere to a stricter set of warnings

[VivekPanyam]

Summary:

(will add summary before requesting review)

Test Plan:

CI +

./build/build.sh --config ci (which treats warnings as errors)

[codecov]

Codecov Report

Merging #439 into cc_warnings will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           cc_warnings     #439   +/-   ##
============================================
  Coverage        89.30%   89.30%           
============================================
  Files              101      102    +1     
  Lines             5536     5537    +1     
============================================
+ Hits              4944     4945    +1     
  Misses             592      592           

Impacted Files	Coverage Δ
source/neuropod/bindings/c/c_api.cc	100.00% <ø> (ø)
...java/src/main/native/com_uber_neuropod_Neuropod.cc	83.81% <ø> (ø)
...d/bindings/java/src/main/native/jclass_register.cc	76.81% <ø> (ø)
source/neuropod/internal/backend_registration.hh	100.00% <ø> (ø)
...urce/neuropod/multiprocess/mq/ipc_message_queue.hh	100.00% <ø> (ø)
...neuropod/multiprocess/mq/ipc_message_queue_impl.hh	91.11% <ø> (ø)
...ource/neuropod/multiprocess/multiprocess_worker.cc	95.45% <ø> (ø)
source/neuropod/neuropod.cc	97.14% <ø> (ø)
source/neuropod/backends/neuropod_backend.cc	96.72% <100.00%> (ø)
source/neuropod/backends/tensorflow/tf_backend.cc	90.94% <100.00%> (ø)
... and 19 more

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[VivekPanyam]

Superseded by #442, #443, and #444