MB-9620: Revert "MB-6413: open all databases on startup"

This reverts commit 7db41d630c782db9eee6cd9e74b3b45324bcf463.

Previously, we needed to open all dbs on startup to make
capi_set_view_manager's use of
couch_server:all_known_databases_with_prefix/1 work reliably. And the
later was needed to avoid adding vbuckets into index too early (before
they're persisted to disk).

Now that indexes are using upr and that capi_set_view_manager doesn't
have code to track on-disk vbuckets anymore, we don't need this logic.

Moreover that logic now becomes harmful w.r.t. memory usage. I.e.
with deactivation of mccouch we can now completely avoid opening
vbuckets in couchdb. And that saves us approximately 100kb of memory
per vbucket.

Change-Id: Icadcd51a542cc006778ae73cb48544adf796992d
Reviewed-on: http://review.couchbase.org/36025
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>