sqlite backend ignores names on reflected foreign key constraints

[Mike Bayer]

this is to complement #3244. While for UNIQUE, we use .schema to get at the unique constraints, therefore we have the names, for FOREIGN KEY we use .pragma which does not give us the names, however .schema does. Foreign key reflection should therefore be rewritten to use parsing of .schema, however at the same time I'd like to keep the .pragma call in place as well as a means to double check what we get back.

this is also critical for Alembic support.

[Mike Bayer]

• rework sqlite FK and unique constraint system to combine both PRAGMA and regexp parsing of SQL in order to form a complete picture of constraints + their names. fixes #3244 fixes #3261
• factor various PRAGMA work to be centralized into one call

→ <<cset 468db416dbf2>>