Sentinel: added SENTINEL MASTER <name> command.

With SENTINEL MASTERS it was already possible to list all the configured
masters, but not a specific one.
anetResolveIP() added to anet.c.

The new function is used when we want to normalize an IP address without
performing a DNS lookup if the string to resolve is not a valid IP.

This is useful every time only IPs are valid inputs or when we want to
skip DNS resolution that is slow during runtime operations if we are
required to block.
Sentinel: SENTINEL MONITOR command implemented.

It allows to add new masters to monitor at runtime.
anetResolveIP() prototype added to anet.h.
Sentinel: flush config on disk when new master is added.