Add flags to page release, respect WT_READ_NO_GEN to avoid forced eviction

FTR, this is what I had in mind instead of #1134.

refs #1128, #1134
@michaelcahill: with WT_READ_NO_EVICT set here, do we still need the test against `(btree->checkpointing && __wt_page_is_modified(page))` in __wt_page_release?