Don't try to set and clear session->split_gen in WT_INTL_FOREACH_BEGIN: there are too many of those loops to ensure that none of them skip clearing it.  Instead, make sure all calls are wrapped in WT_WITH_APAGE_INDEX.

refs SERVER-16546
Set split-gen when deepening the tree, add paranoia that the eviction server doesn't keep split-gen pinned.
Merge pull request #1496 from wiredtiger/server-16546

Don't try to set and clear session->split_gen in WT_INTL_FOREACH