Now, as we don't need to take the schema lock for checkpointing, it is
only used to synchronize concurrent space modifications (drop, truncate,
alter). Actually, a global lock is a way too heavy means to achieve this
goal, because we only care about forbidding concurrent modifications of
the same space while concurrent modifications of different spaces should
work just fine. So this patch replaces the global schema lock with per
space locking.

A space lock is held while alter_space_do() is in progress so as to make
sure that while AlterSpaceOp::prepare() is performing a potentially
yielding operation, such as building a new index, the space struct
doesn't get freed from under our feet. Note, the lock is released right
after index build is complete, before the transaction is committed to
WAL, so if the transaction is non-yielding it can modify the space again
in the next statement (this is impossible now, but will be done in the
scope of the transactional DDL feature).

If alter_space_do() sees that the space is already locked it bails out
and throws an error. This should be fine, because long-lasting operation
involving schema change, such as building an index, are rare and only
performed under the supervision of the user so throwing an error rather
than waiting seems to be adequate.

Removal of the schema lock allows us to remove latch_steal() helper and
on_begin_stmt txn trigger altogether, as they were introduced solely to
support locking.

This is a prerequisite for transactional DDL, because it's unclear how
to preserve the global schema lock while allowing to combine several DDL
statements in the same transaction.