Minor code aesthetic change to use Redis code base style rule of saving vertical space when possible.

Removed handling of deprecated hash-max-zipmap-entries nad hash-map-zipmap-value. Pieter is too good with users ;). Better to have them switch to a saner configuration ASAP after the 2.6 upgrade.

Added a top-function comment to rioWriteHashIteratorCursor() to better specify what the function does. Not immediately clear from the name.

The Run ID is a field that identifies a single execution of the Redis
server. It can be useful for many purposes as it makes easy to detect if
the instance we are talking about is the same, or if it is a different
one or was rebooted. An application of run_id will be in the partial
synchronization of replication, where a slave may request a partial sync
from a given offset only if it is talking with the same master. Another
application is in failover and monitoring scripts.

clusterGetRandomName() generalized into getRandomHexChars() so that we can use it for the run_id field as well.

By default Redis refuses writes with an error if the latest BGSAVE failed (and at least one save point is configured). However people having good monitoring systems may prefer a server that continues to work, since they are notified that there are problems by their monitoring systems. This commit implements the ability to turn the feature on or off via redis.conf and CONFIG SET.

Redis now refuses accepting write queries if RDB persistence is
configured, but RDB snapshots can't be generated for some reason.
The status of the latest background save operation is now exposed
in the INFO output as well. This fixes issue #90.

Better MONITOR output, now includes client ip:port or the lua string if the command was executed by the scripting engine.

anetPeerToString() automatically populates ip/port with something that may be provided to the user as output in case of errors.

The new code uses a more generic data structure to describe redis operations.
The new design allows for multiple alsoPropagate() calls within the scope of a
single command, that is useful in different contexts. For instance there
when there are multiple clients doing BRPOPLPUSH against the same list,
and a variadic LPUSH is performed against this list, the blocked clients
will both be served, and we should correctly replicate multiple LPUSH
commands after the replication of the current command.

Added a new API to replicate an additional command after the replication of the currently executed command, in order to propagte the LPUSH originating from RPOPLPUSH and indirectly by BRPOPLPUSH.

Fix the build on Mac OS X, where features.h is non-existent

Simple install script changes

Fix for issue #306, thanks to tchajed (on github) for the pull request. The original patch was reworked a bit.