in_host_list: 3 changes:

1. read in config file in register, with a callback to reparse
   a) config file refreshes when altered
   b) regex only recompiled when changed (was every transaction)

2. when relaying=true, rcpt gets an automatic pass
   (before: relaying connections rejected at rcpt)

3. when relaying=true, mfrom host validated against in_host_list
      because by default, that's the sensible thing to do
host_list: documented behavior when relaying=true
in_host_list: load config at register with callback,

for relaying clients, only OK outbound recipients when MAIL FROM domain is local
in_host_list: yummy, lotsa delicious tests

Running tests:  [ 'tests/plugins/rcpt_to.in_host_list.js' ]

rcpt_to.in_host_list.js
 in_host_list - miss
 in_host_list - hit
 in_host_regex - undef
 in_host_regex - miss
 in_host_regex - exact hit
 in_host_regex - re hit
 hook_mail - null sender
 hook_mail - miss
 hook_mail - hit
 hook_mail - hit, regex, exact
 hook_mail - hit, regex, pattern
 hook_rcpt - missing txn
 hook_rcpt - hit list
 hook_rcpt - miss list
 hook_rcpt - hit regex, exact
 hook_rcpt - hit regex, pattern
 hook_rcpt - miss regex, pattern
 hook_rcpt - rcpt miss, relaying to local sender

OK: 33 assertions (117ms)
host_list: improved and simplified config.get syntax
tests/stub_conn: add empty notes object

avoids plugins having to set it up for themselves dozens of times
host_list: ws changes
Merge pull request #551 from msimerson/host_list

rcpt_to.in_host_list, handle connection.relaying use case