Clarify bootstrap API layout

This removes all the helper macro entry points into
lcb_bootstrap_common() and replaces them with direct calls. Additionally
it creates a new file called bootstrap.h which contains a more detailed
description of how bootstrap actually works, and is now indexed by
Doxygen via `make doc-internal`.

Change-Id: Ice53f3b34524ffb41ee0cb58897098ceaefc5206
Reviewed-on: http://review.couchbase.org/40014
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>