Supports setting the read lock mode in R/W transactions at both the client level and at an individual transaction level.

This change allows for the read lock mode to be set independently or alongside the transaction isolation level selection. Therefore transactions can be run in the following modes:

• Serializable isolation with pessimistic locking (default isolation level and locking mode in Spanner)
• Serializable isolation with optimistic locking
• Repeatable read isolation with optimistic locking (default locking mode for Repeatable Read isolation)
• Repeatable read isolation with pessimistic locking

If no isolation level is selected, Spanner transactions default to Serializable isolation with pessimistic locking. If no read lock mode is chosen for Repeatable Read isolation, transactions default to optimistic locking.