ISPN-14641 MULTI + ISPN-14643 EXEC

[jabolina]

https://issues.redhat.com/browse/ISPN-14641
https://issues.redhat.com/browse/ISPN-14643

This was the easy part. Working fine in a single node/STANDALONE. This way, we have a few things already:

• The commands are in memory, not tolerating restarts;
• TX could be half-applied if crashing mid-way EXEC;
• Executing the queued commands has a read uncommitted isolation.

Going to CLUSTER mode becomes more difficult. We lose the transaction context, as it is bound to a channel. We can start a discussion about how we want to handle all this. But for now, we have at least the basics in.

[karesti]

@jabolina done!