CCBC-452: Rename 'DSN' to 'CONNSPEC' and 'connstr'

This is a makeover of the naming semantics within the previously termed
"DSN" subsytem. The string input parameter is referred to now as
'connstr' whereas the compiled structure is known as 'lcb_CONNSPEC'.

Change-Id: I527cebc64a73a8bb3ee22c8c1fcbca68be4969f5
Reviewed-on: http://review.couchbase.org/38770
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>