simplify background writer thread implementation, tying the sync object lifetime more closely to the thread object lifetime

    MySQL compatibility: support REPLACE INTO as an alias for MERGE INTO.
it has recently been implemented

Issue 517: Create or replace view statement has no effect on the others already existing JDBC connection    

Issue 518: java.sql.Connection.commit() freezes after LOB modification with EXCLUSIVE connection    

Improve error message when dropping an index that belongs to a constraint, specify constraint in error message.

MVStore: closing a map and making it read-only is now only a marker and not enforced, to allow concurrent store operations