Session commands that span multiple packets are now allowed and will
work. However, if one is executed the session command history is disabled
as no interface for appending to session commands exists.
The backend protocol modules now also correctly track the current
command. This was a pre-requisite for large session commands as they
needed to be gathered into a single buffer and to do this the current
command had to be accurate.
Updated tests to expect success instead of failure for large prepared
statements.
Moved the RWBackend class implementation into its own file. Made some of
the command type functions a part of the <maxscale/protocol/mysql.h>
header to make it reusable.
