Add references from Client and Statement to each other

[sodabrew]

This is to prevent out of order GC. Because GC of a Statement object generates server commands to release the statement, the client connection can be in the middle of some other operation like streaming a result set that doesn't allow other commands.

Add a new accessor Client#prepared_statements that returns the list of all outstanding statements. This is sort of a nice-to-have that falls out of having the Statement object references in the Client object.

This is not a complete fix yet. It crashes under large numbers of objects. Also need to document if/why the list of Statements should be/not be freed when the Client connection is closed. I think the server will automatically release the prepared statements when the connection drops, but I'm not sure.

Fixes #1043.