Monitor attachment fd during GET

This prevents download interruption if the LRU closes the associated
database. Will need a tweak when external attachments land in master.

BugzID: 16492
Validate attachment names are UTF-8

BugzID: 16929

Conflicts:
	src/chttpd_db.erl
Fix 500 on _changes call to missing db

Handle an immediate error on changes call (one that doesn't call
start, so doesn't start the req -> delayed_resp change)

BugzID: 16565
Send error if normal startup fails too

BugzID: 16942