Informs cockroachdb#22329.

This commit adds the max_prepared_transactions session variable, returning a
value of math.MaxInt32 to indicate that there is no limit on the number of
prepared transactions that can be created.

This change has the effect of unblocking the pgjdbc XADataSourceTest test
suite, which was previously failing during initialization. Is also includes
a small patch to work around cockroachdb#31632 (unsupported deferrable qualifier) so
that each test in the XA test suite actually runs (and fails).

Release note: None

Informs cockroachdb#22329.

This commit adds a new `system.prepared_transactions` table, which will
be used to track prepared transactions. The table includes all of the
information needed to implement XA two-phase commit transactions, along
with additional metadata needed to implement `pg_catalog.pg_prepared_xacts`.

Release note: None

Informs cockroachdb#22329.

This commit adds new syntax for `PREPARE TRANSACTION <gid>`,
`COMMIT PREPARED <gid>`, and `ROLLBACK PREPARED <gid>`, for
parity with Postgres support.

Handling of the syntax is currently unimplemented.

Release note: None

Informs cockroachdb#22329.

This commit adds support for Txn.Prepare, DB.CommitPrepared, and
DB.RollbackPrepared APIs to KV. These methods allow clients to prepare a
transaction for commit, commit a prepared transaction, and rollback a
prepared transaction, respectively.

When a transaction is prepared, we send an EndTxnRequest as usual, but a
new flag indicates the transaction should be moved to status PREPARED
instead of COMMITTED. In state PREPARED, a transaction record is updated
to include all of its intents, and awaits a subsequent EndTxnRequest
which will move the transaction either to COMMITTED or ABORTED.
Transaction in state PREPARED cannot be pushed, regardless of isolation
level, ensuring that a subsequent commit or rollback is guaranteed to
succeed.

Release note: None