v0.18.1

Note this is the equivalent of v0.20.0, but still using CE2.

This release makes a change to the way we process messages from Pub/Sub which should considerably improve performance.

Changes from v0.18.0:
#330 Emit larger chunks in HTTP and gRPC consumers (based on @istreeter's #329, thanks @bastewart)

Note this change as a few caveats, please let us know if you experience any issues with this:

• In both consumer variants if any message in a chunk fails to
  decode the errorHandler is invoked before any successfully
  decoded messages in the chunk are emitted downstream; even
  if the message which failed to decode comes after the successful
  ones.
• In the gRPC variant alone the stream may fail sooner if an internal
  failure occurs in the Java library. This should not matter at all.
• The user-provided errorHandler callback will now get called for all
  messages in the chunk before the chunk gets emitted downstream