Correct locking.

[addiks]

Currently there is no locking at all. For in-memory databases (without threads) this is not a problem, but it will become a problem when the database is saved on the actual filesystem.

There must be a way to lock tables or rows.

Implementation plan: While the table- and column-data- implementations work with the actual files, the executors use abstract data-providers (filters, orderers, ...) that themself use arbitrarely deep nested the actual implementations. That means we need an Interface (LockableInterface) that indicates that this data-provider can be locked. The column-data/table/schema implementations will do actual locking and the other data-providers will just forward the lock-operations. Maybe have the actual locking done by a seperate class of components (Lockers?) that can depending on the implementation either just use the 'flock' functions or use more complex semaphores.