Prevents sequence writing from becoming a bottleneck when doing bulk load.

We already have the MAX_MEMORY_ROWS setting which does a very similar thing, and is better documented.

In server mode, appending ";autocommit=false" to the database URL was working, but the return value of Connection.getAutoCommit() was wrong.

Issue 558: with the MVStore, a NullPointerException could occur when using LOBs at session commit (LobStorageMap.removeLob).