make filename a constant

Changing the file extension means that syntax highlighting make the contents prettier :-)

the link() and unlink() methods from Value are only called from the backend, so pass in the Database instance.
That lets me remove the removeLob() method from LobStorageInterface.

rename PageStore#systemSession to PageStore#pageStoreSession, to distinguish it from Database#systemSession

LobStorageBackend is only ever constructed from Database, so let's use it directly