Fix bugs with couch_proc_manager limits

This fixes the couch_proc_manager limit counting by rearranging the increment and decrements when processes are created and destroyed. It ensures that each time we remove a process from the ets table that we decrement appropriately.

For incrementing things are a bit more complicated in that we need to increment before inserting to the table. This is so that our hard limit applies even if one of our asynchronous spawn calls is opening a new process. This is accomplished by incrementing the counter and storing the async open call information in a new ets table. If the open is successful the counter is left untouched. If the open fails then we need to decrement the counter.

This also simplifies starting waiting clients when a processes is either returned, exits, or fails to start by isolating the logic and calling it in each place as necessary.

Closes COUCHDB-2321