Introduce execute_many/3

[fishcakez]

Adds an efficient method of executing multiple queries inside a transaction.

• Automatically adds a transaction around the call (disabled with transaction: false)
• Executes many queries (pipelined in postgrex's case for 1 round trip)
• Stops executing on first error and just returns the error
• If inside a transaction rolls back the transaction on error
• All decoding delayed until after the transaction is committed (potentially after checking in the connection)
• All logging delayed until after the transaction (potentially after checking in the connection)

This means we can execute any number of queries inside a transaction and not block the connection at all to log or decode. Also in postgrex it would only take 3 round trips, however many queries are executed, when wrapping with a transaction.

The caveat is that queries must be prepared beforehand.

Potential issues:

• Ignores results of successful queries if there is an error
• decode_mapper breaks down as its set for all queries, should the API be extended to set per query options? Would that make this too complicated to use?
• Do we want to commit a transaction we cant decode?

[fishcakez]

There are no plans to use this at present.