Split message aggregators

Split message aggregators

This splits the EOM aggregator and the chunked encoding aggregator. The
chunked encoding can be much more efficient and hardened as it allows
for proper stream processing.

We do not go the entire way here, just tighten the chunk header parsing
by keeping an explicit state machine. We still need to gather the entire
message before shifting it, as our downstream expects to see the message
in its entirety.

Change-Id: I60947b6d018b73581a56adda5212d3482dda61ea
Signed-off-by: Robert Varga <rovarga@cisco.com>