http: speed up shutdown

http: speed up shutdown

This continues/fixes #6719.

`event_base_loopbreak` was not doing what I expected it to, at least in
libevent 2.0.21.
What I expected was that it sets a timeout, given that no other pending
events it would exit in N seconds. However, what it does was delay the
event loop exit with 10 seconds, even if nothing is pending.

Solve it in a different way: give the event loop thread time to exit
out of itself, and if it doesn't, send loopbreak.

This speeds up the RPC tests a lot, each exit incurred a 10 second
overhead, with this change there should be no shutdown overhead in the
common case and up to two seconds if the event loop is blocking.

As a bonus this breaks dependency on boost::thread_group, as the HTTP
server minds its own offspring.

Zcash: cherry-picked from commit a264c32e3321ae909ca59cb8ce8bf5d812dbc4e1