Add a new mode to eviction that only considers internal pages.

The mode is triggered when we are force evicting pages, and the cache
is not yet full. If we don't merge internal pages while doing forced
eviction we can end up with very deep chains of WT_REC_SPLIT pages.
whitespace
Merge pull request #843 from wiredtiger/evict-internal-forced

Add a new mode to eviction that only considers internal pages.