MagmaEncryption: Stop using old key immediately upon new key configuration

Not a P0 requirement yet, but ideally once SetCurrentKey is called (to set a new master key), we should immediately start using it.

This means any append writers/wal segments open must be forced to close and new ones must be started.

Inside of BeginTxn, we can add checks to see if master key has changed and accordingly close current segment and start a new one. 

Similarly, inside of LSMTree flush, we can do the same check and terminate the append writer and start a new one.