Rename verbose ckpt to checkpoint, add some more messages to it.

The messages relate to the volume of data being written in checkpoints.
Merge pull request #923 from wiredtiger/verbose-checkpoint

Rename verbose ckpt to checkpoint, add some more messages to it.
Merge branch 'develop' into new-split

Conflicts:
	src/btree/bt_evict.c
	src/include/flags.h