Sqlite: check anchor to tx foreign key constraint

[notmandatory]

"In the SQLite persistence the foreign key constraint from anchor to transactions is not checked. Adding an anchor for a non-existing transaction will prevent persisting the wallet."

[notmandatory]

Is there a scenario where were need to insert an Anchor without a corresponding transaction id?

[ValuedMammal]

We should also revisit #961 when trying to reproduce the SQL error.

[notmandatory]

@oleonardolima is taking a look at this one. Or possibly just helping with testing @evanlinjin's PR #1736.