sql: display error on FK creation and drop failure

Before insertion to _fk_constraint we must be sure that there is no
entry with given <name, child id>. Otherwise, insertion will fail and
'duplicate key' will be shown. Such error message doesn't seem to be
informative enough, so lets verify before insertion to _fk_constraint
that it doesn't already contain entry with given name.
The same is for dropping constraint, but here vice versa: we test
that _fk_constraint contains entry with given name and child id.

It is worth mentioning that during CREATE TABLE processing schema id
changes and check in OP_OpenRead opcode fails (which in turn shows that
pointer to space may expire). On the other hand, _fk_constraint space
itself remains immutable, so as a temporary workaround lets use flag
indicating pointer to system space passed to OP_OpenRead. It makes
possible to use pointer to space, even if schema has changed.

Closes #3271