Synchronize rsmgclient with pymgclient

[antaljanosbenjamin]

As we have multiple clients (pymgclient, rsmgclient, the nodejs client) it would be nice if they were been implemented in the same way. I think making a C library on top of mgclient is not useful, because of the various differences in the different languages the API and wrapping can be complicated. Therefore I propose to use the same state machine logic for all clients. I think the biggest difference between pymgclient and rsmgclient currently is how the transactions are handled:

• for pymgclient there are different states for READY and IN_TRANSACTION
• for rsmgclient there is a bool flag that indicates whether the connection is in a transaction or not

The smaller differences are:

• in pymgclient in most (if not every) function that changes the state of the connection documents the preconditions (the state and value of some flags, i.e.: autocommit) with asserts. For rsmgclient I think this is not that consistent. Also, the these checks are using blacklisting instead of white listing.
• minor differences in naming: Connection::pull_all also fetches the results (pull -> tell the server to prepare some results, fetch -> get the results of a previous pull)
• in pymgclient there is different cursor and connection class, while in rsmgclient there is only a connection. I think this is fine and no change is required because of this.