get rid of max_dbs_open

we don't need it.
stop maintaining LRU
open databases synchronously

Fix the issue we see when running couch_index/couch_mrview when a database is
opening while just beeing deleted. Also we don't need to open the database
asynchronously.
just store the pid in couch_server
fix exit
fix couch_server terminate/2

since we only store the pid now, make sure to match it.
couch_dbs_by_name is an ordered set
fix error when the db exists
bump to 1.6.2