proxy: redo libevent handling code

The event handling code was unoptimized and temporary; it was slated for
a rewrite for performance and non-critical bugs alone. However the old
code may be causing critical bugs so it's being rewritten now.

Fixes:
- backend disconnects are detected immediately instead of on the next
  time they are used.
- backend reconnects happen _after_ the retry timeout, not before
- use a persistent read handler and a temporary write handler to avoid
  constantly calling epoll_ctl syscalls for potential performance boost.

Updated some tests for proxyconfig.t as it was picking up the
disconnects immediately.

Unrelated to a timing issue I resolved to the benchmark.

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