WiredTiger currently supports read-uncommitted isolation, which allows reads of data that may be rolled back, and snapshot isolation, which provides strong read consistency but aborts transactions that attempt to update items that were updated by a concurrent transaction.

Add support for read-committed isolation, which provides some level of read consistency: essentially, a snapshot that is periodically updated as the transaction runs. This is equivalent to SQL Server's "snapshot read committed" isolation level.

Further, move the cursor "isolation" setting into the session handle, and support read-committed isolation for non-transactional operations. The session-level isolation level will default to read-committed, and be the default level for transactions where no isolation level is specified.