Refactor OS process management

BugzID: 11572

See also COUCHDB-901

A new config setting is introduced.  The following block controls the
maximum number of OS processes that will be reused.  Additional OS
processes will still be spawned on-demand, but they'll be terminated
when the clients are through with them.

[query_server_config]
os_process_soft_limit = 100
Remove a function clause that can never match
Do not start couch_log event handler
Restore some useful logging re: opening new DBs
Hibernate after update_docs to reduce memory footprint
Ignore key in doc.views if value is not an Object

BugzID: 11864
API for triggering view updates
Make couch_view_updater amenable to hot upgrades