Problem: discovery interval too high for wired LAN

Default setting is 1,000, which is OK for WiFi but too slow for wired
LAN, where UDP broadcast packets are very cheap.

Solution: expose this as zyre_set_interval() so calling application
can tune for specific use case. For wired LAN, 250 msec should be fine.
Simplified zyre_log_connect API

- afaics we don't need the separate methods for subscribers
- I switched to using a hash table for efficiency
- I removed the disconnect method, which was not orthogonal to
  connect, and doesn't solve any identified problem afaics
- some compile warnings removed along the way
Merge pull request #171 from hintjens/master

Fixed up zyre_log_connect/destroy