Sentinel: don't write HZ when flushing config.

See issue #1419.
Grammar fix in freeClient().
Removed old comments and dead code from freeClient().
BLPOP blocking code refactored to be generic & reusable.
blocked.c API commented.
WAIT command: synchronous replication for Redis.
Fix clients timeout handling.

During the refactoring of blocking operations, commit
82b672f6335ac2db32a724ba5dc10398c949a4a8, a bug was introduced where
a milliseconds time is compared to a seconds time, so all the clients
always appear to timeout if timeout is set to non-zero value.

Thanks to Jonathan Leibiusky for finding the bug and helping verifying
the cause and fix.
Fixed typos in redis.conf file.
Fixed grammar: before H the article is a, not an.
Fixed grammar: 'usually' to 'usual'
Merge pull request #1437 from AnuragRamdasan/patch-1

Fixed grammar: 'usually' to 'usual'
fixed typo
Merge pull request #1438 from AnuragRamdasan/patch-2

fixed typo
Grammar fix.
Merge pull request #1439 from AnuragRamdasan/patch-3

Grammar fix.