Reduce retained references to ConnectionContext

[mirromutth]

Motivation:

A small refactoring of ConnectionContext usage.

It is based on challenges #228 and #89. If a MySQL connection is reconnected or redirected, the ConnectionContext needs to be recreated. If we accidentally retain a reference to ConnectionContext outside the client package, it may cause some unexpected behavior.

Of course, there are some counterexamples here, such as MySqlDataRow that needs to retain the CodecContext in which it was responded to. Regardless of whether the connection changes, the CodecContext should be always the context of the connection which was responding to this row, instead of the current connection.

Modification:

• Reduce publicity
• Prepare to support redirect and reconnect

Result:

Remove ConnectionContext reference that's retained outside client package, except those required by codecs

[jchrys]

LGTM