Introduce `TransactionMode`

[honzasp]

The client has been using BEGIN DEFERRED as the transaction mode, which is almost never the most useful option:

• For readonly transactions, it prevents them being executed on the replica
• For read-write transactions, it causes random transaction failures if there is a concurrent write transaction on the server.

This PR introduces TransactionMode, which allows the user to select one of these options:

• "write" does BEGIN IMMEDIATE, so that write transactions are serialized instead of aborted
• "read" does BEGIN TRANSACTION READONLY, so that sqld can optimize it as read-only (this syntax is not yet recognized and treated as BEGIN DEFERRED by sqld)
• "deferred" explicitly opts into BEGIN DEFERRED.