sql: rework parsing of transaction statements

This patch reworks the parsing of statements associated with
transactions. There are six of them:
START TRANSACTION;
COMMIT;
ROLLBACK;
SAVEPOINT savepoint_name;
RELEASE [SAVEPOINT] savepoint_name;
ROLLBACK TO savepoint_name;

Prior to this patch, VDBE for these statements were generated while the
statement was in the process of being parsed. After this patch, VDBE for
all these statements will be generated only after the parsing has
successfully completed.

Part of #5485

NO_DOC=Will be added later
NO_TEST=No user-visible changes are made
NO_CHANGELOG=Will be added later