Preload internal pages into file system cache with compression.

Previously the functionality only worked without compression enabled.
Merge branch 'develop' into warm-cache-compress

Conflicts:
	src/include/flags.h
Create a more general-purpose block-manager preload call and pre-load
individual pages instead of using the page locations to guess at the
right file chunks.
Merge pull request #559 from wiredtiger/warm-cache-general

Create a more general-purpose block-manager preload call
Merge pull request #538 from wiredtiger/warm-cache-compress

Preload internal pages into file system cache with compression.