Remove locking from the query path

The Tx lock in Tx.Property() was hit during the LUA query generation when the schema is queried for available variables. The lock shouldn't be necessary as long as the schema isn't changing on the fly.

The DB.OpenTable() was locking for too long because the table cache wasn't hit for tables that are already open. Fixing that pretty much eliminates blocking time on that.

@benbjohnson 

/cc @snormore, @charlescng