Optionally skip block cleanup and leak memory on connection close.

@agorrod, I think this does what you wanted, if you agree, please merge and close #845.

One thing that occurred to me: we could have `__wt_free_int()` check the new connection flag and not free any memory once we've started closing the connection, but my concern is we might slow down the common free path with an additional main memory fetch, and that seemed like a bad idea.  Do you agree, or is an additional memory fetch in the free function no big deal, sine we're about to call the underlying library free function anyway?  @fedorova, any thoughts?