Default fam blocking to true

Based on discussion in #bcfg2, the consensus seems to be that the
behavior provided by fam_blocking = True is the least surprising
of the two options (i.e. the server should not process data until
it is ready).  1.4 seems like a good time to make this change.

There is a quirk in the configuration parser where a BooleanOption
with default=True will always return false __unless_ a command-line
option is provided.  Not sure why that's the case, but this is a work-
around
This line isn't necessary -- `setdefault()` already checks for the key in the dict. :)