First pass at parsing new socket encoding scheme

New parsing logic can be turned on using the DNEW_ENCODING_SCHEME flag.
Currently, we do not support parsing annotations or errors.

To simplify the LogEvent constructor, we remove the creation of
log_msg objects within StatsSocketListener. This change to the LogEvent
constructor also forced us to modify the LogEvent benchmarking code.

Test: m -j128
Test: bit statsd_test:* (passes when flag is off)
Test: atest StatsdHostTestCases (passes when flag is off)
Test: bit statsd_benchmark:*
Change-Id: I827b72f46a617dbc5194ad778fcf7c3d794efb7b