Support prepared statements (PQ)

[pixelspark]

Support prepared statements over the Postgres wire interface.

More details: https://www.pgcon.org/2014/schedule/attachments/330_postgres-for-the-wire.pdf

• [x] Parse message => ParseOk
• [x] Bind message (sends BindComplete) => BindOk
• [x] Execute message => RowDescription, DataRow... CommandComplete/PortalSuspended
• [x] Close message (for portals and prepared statements)
• [x] Flush message (/PortalSuspended?)
• [x] Describe message
• [x] Sync message => ReadForQuery
• [x] Support binding of parameters ($1, $2, ..) in queries ==> generic 'PreparedStatement' interface?

At this point it would also make sense to put the query server in a separate package so others can benefit as well.