daemon: dedupe PublicInbox::Config objects by pathname
This means all Inbox, Git, Over, Msgmap, Search objects also get
deduplicated if they belong to the same config file, reducing
memory and FD usage. This helps save memory and improve cache
hit rates in -netd setups where NNTP, IMAP, HTTP, and POP3
servers run in the same process.
InboxIdle was the only bit which needed adjustment, but there
may be other bugs lurking despite all tests passing.