paginate all_dbs

simple way to paginate results by skipping first results and limit the
number of results returned.

fix #COUCHDB-1960
descriptive name for the maximum number of dbs returned.
doc: add the all_db pagination feature to the 1.6 news
rename MAX_DBS define to MAX_DBS_LISTED

per suggestion of @kxepal rename it for the code readers.