Stop counting handlers

The transport programs (client, server, and server-webrtc) have some obsolete handler-counting code that tries to keep the process alive until all ongoing connections have terminated. This is a remnant of an earlier version of pt-spec that prescribed different handling of SIGINT and SIGTERM, and required this behavior on SIGTERM. pt-spec has since changed to no longer mention SIGINT, and prescribe immediate shutdown (terminating ongoing handlers) on SIGTERM.

Here's background: comment:5:ticket:26389.

Here's the corresponding change in meek: https://gitweb.torproject.org/pluggable-transports/meek.git/commit/?id=c7541223c704f76cb45a4e20bd20d963ea8d1fc7