Update CBSASL distribution

This largely adapts the upstream cbsasl distribution. The big changes
are the removal of the dependence on the internal "cbsasl.h"; which
makes the distribution slightly smaller and eliminates the use of custom
"cbsasl_XXX_t"

Change-Id: Ie0cbea4af550c6d52f05f5cb03ec8bb57061523e
Reviewed-on: http://review.couchbase.org/43793
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>