Use pull model to get the statistics from DohFrontend

As we are adding more fields in Stats, usig lock would become more
expensive. Change DohFrontend to fetch the stats by mpsc channel.

Besides, this change also contains some cleanups and minor fixes:
- Replace mpsc::channel with mpsc::unbounded_channel
- Rename connections to connections_accepted in Stats
- Change the return type of ClientMap::get_or_create()

Bug: 1895315
Test: cd packages/modules/DnsResolver && atest
Change-Id: I184d45c148b15389af26ad482b947def0a222e6a