Doomsday Engine

Optimize|libdeng2|Lockable: Use automatic storage with Lockable

Lockable, as used by Guard, should not use the current pimpl idiom
implementation as this allocates storage for the private instance
from the heap.

The multithreaded logging system makes frequent use of Guards for
ensuring concurrency robustness. This inadvertently resulted in a
significant and unnecessary perf bottleneck.