Add a WT_SIZE structure cache to the WT_SESSION_IMPL handle.

Create a cache of WT_SIZE structures in the WT_SESSION_IMPL handle so we (hopefully) never call malloc while holding the block manager's live lock, and minimize modifying the WT_BLOCK structure's cache lines.

@michaelcahill, here's the parallel change to cache WT_SIZE structures in the same way we're now caching WT_EXT structures.