sql: Support persistency for SQL triggers

Persistency for SQL triggers supported by introducing
new system space called `_trigger` which should store all
SQL triggers registered. Add space itself, make
visible to Lua, update bootstrap code and regenerate
bootstrap.snap.
Insertion into the space is done by introducing new VDBE
opcode `OP_ParseSchema3` which provides a pair of trigger
name and SQL which creates the trigger.
During system restore `_trigger` is scanned and its contents
parsed back into VDBE.
Update tests and extend test for persistency w/ trigger case.

Closes #2320