Fix false positive in memory efficiency test.

Fixes issue #1298.
Sentinel: fix type specifier for Hello msg generation.

This fixes issue #1395.
Sentinel: fixes inverted strcmp() test preventing config updates.

The result of this one-char bug was pretty serious, if the new master
had the same port of the previous master, but just a different IP
address, non-leader Sentinels would not be able to recognize the
configuration change.

This commit fixes issue #1394.

Many thanks to @shanemadden that reported the bug and helped
investigating it.