PG wire improvements. Fixes #450

[jaugsburger]

This change deals with issues around protocol mechanics, in particular our responses to messages from the client on simple and extended mode.

For example:

• we now wait for sync message to send reply back, therefore grouping replies. This means that if client sends messages: P/B/D/E/S, we now send one reply back with 1/2/T/D/C/Z. This means that there is less chatter on the wire because 1. we are sending less messages and 2. the client does not get confused and therefore send less messages.
• for each message sent by client, we send an acknowledgement back
• stop sending spurious messages
• named prepared statements are cached on parse and we correctly bind parameters when receiving named statement on the Bind and Describe messages
• when executing 'select', we now send back the number of returned rows
• when executing 'set', we now send correct query tag (SET)
• for empty queries, we reply back with correct message type
• for queries with no data, we reply back with 'no data' message type
• row description now includes column number
• row descriptions attempts to populate type size and type modifier

[CLAassistant]

All committers have signed the CLA.