Updated forestdb to latest master

(Was previously on 1.0b branch)
Removed workaround for fdb_iterator_seek not being able to go backwards

Now that MB-11470 is fixed.
Added a default logging callback that writes to stderr.

To disable logging, explicitly set LogCallback to NULL.
Fixed a problem in alloc_slice -- shouldn't be convertible to bool

The shared_ptr base class wasn't private, so it exposed some accessors
I don't want, including one that allowed alloc_slice to be tested as a
boolean.
Added 'descending' option for doc/index enumerators.

Implementation is still a bit hacky until forestdb fixes MB-12465.