MB-10220 Always allow generate_map_chain use chain maps.

Since we started using generate_map_chain for generation of star maps
for the case when num_replicas is 1 and no tags are defined, the code
there should be adapted to accept past maps with 'chain'
replication_topology even when current replication_topology (and hence
replication_topology in options passed to the function) is 'star'.

Change-Id: I989d28618f96f4af09c9b78eda7e3ee97f366dea
Reviewed-on: http://review.couchbase.org/33684
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Fix dialyzer warnings.

Change-Id: I2149af37833fae58ddea05011a7aca0d426b8074
Reviewed-on: http://review.couchbase.org/33687
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Don't do HEAD request in couch_api_wrap:db_open.

Change-Id: Ie6a0154c9fd2a2d2cafb8b3acb1552bf1c3bca13
Reviewed-on: http://review.couchbase.org/33692
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>