Update code for ponyc 0.28.0

  - updates PosixDate format calls to be partial and wrapped in
    try blocks

Mark connection as readable and read data on connect/reconnect

Prior to this commit the CustomTCPConnectionHandler logic did not
set the connection as readable and/or try to read data when a new
connection or re-established connection occurred. This resulted in
the behavior seen in WallarooLabs/wally#2281
as part of the 0.5.0 release of Wallaroo.

This commit properly sets the connection as readable and tries to
read data on a connect/reconnect. It also combines the connect and
reconnect logic into a single if statement to avoid duplicating
the same logic to make maintenance a bit easier.

Fix incorrect partial message detection threshold

Prior to this commit, the logic to determine that a Kafka broker
had returned a partial message was incorrectly calculating and
checking for how much data needed to be available to decode a
message. This would result in an "error decoding remaining
message bytes" error if the size of the partial message was
less than 4 bytes of the complete message.

This commit resolves this bug by properly calculating and
checking for how much data needs to be available for decoding
a message.

Fix invalid throttling edge case

Prior to this commit, if a broker connection that didn't own any
partitions was disconnected, it would cause producers to get
throttled even if no topics/partitions were actually throttled.

This commit fixes the issue.

Compatiblity with ponyc master and exhaustive match changes.

Update pony_asio_event_create match ponyc 0.19.1

pony_asio_event_create signature needed to be updated
for compatibility with 0.19.1.

Shutdown CustomTCPConnectionHandler whenever requested

Previously, if we asked a CustomTCPConnectionHandler to shutdown, it would
only shutdown if it wasn't receiving data. This PR updates
CustomTCPConnectionHandler's close() to always do a hard close and
immediately shut down the connection.

Add Code of Conduct, Contributing, and Readme (#2)