Handle empty list of id/revs in fabric_doc_missing_revs:go
Handle empty list of id/revs in fabric_doc_missing_revs:go

When AllIdsRevs is an empty list the result will always be an empty
response. Handle this explicitly given there is no work to do
(attempting to delegate to the shards previously resulted in a timeout).

This closes #10

COUCHDB-2531

Signed-off-by: Alexander Shorin <287dfd97d2274fe201ae7191a244ba25e475e13a@apache.org>

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb