Removes dofile() from Lua

Use the COMMAND output to fill with partial information the built-in
help. This makes redis-cli able to at least complete commands that are
exported by the Redis server it is connected to, but were not available
in the help.h file when the redis-cli binary was compiled.

This reverts commit 1189a4ea.

Actually this is wrong, the command can be called without args at all.

Update global protection error message

Fix a possible race condition of sdown event detection if sentinel's connection to master/slave/sentinel became disconnected just after the last PONG and before the next PING.

fix linux compile Error zmalloc.c:109: error: invalid lvalue in unary `&'

Fix nanosecond conversion

This fixes a bug introduced by d827dbfd, and makes the code consistent
with the logic of always allowing, while the cluster is down, commands
that don't target any key.

As a side effect the code is also simpler now.

Fixes a typo in the comments

Fix for #3076 Reverse redirect address parse

Thanks to @tushar2708 for the PR. I applied a slightly different fix.
Thanks to @cespare for reporting.

Close #3024
Close #3020

Fixes #2515.

maxmemory_policy fix for #3187

Fixed typo in README.md

Fix redis-trib.rb

Sometimes Redis patch releases are released in a matter of weeks or days
one after the other. In order to have less release friction the idea is
to stop writing changelogs by hand, in order to also cover everything
interesting there is to say. Useless things can be deleted manually by
the changelog. Also this gives more credits to contributors since often
in the commit message involved people are cited even when they are not
the authors of the commit.

Thanks to @oranagra for the idea of allowing CONFIG GET during loading.

Thanks to @oranagra for the hint about misplaced 'r' flags.

Minor fixes found during merge and code review

Thanks to @tidwall for reporting.
Close #3194.

The test works but is very slow so far, since it involves resharding
1/5 of all the cluster slots from master 0 to the other 4 masters and
back into the original master.

This fixes issue #3043.

Before this fix, after a complete resharding of a master slots
to other nodes, the master remains empty and the slaves migrate away
to other masters with non-zero nodes. However the old master now empty,
is no longer considered a target for migration, because the system has
no way to tell it had slaves in the past.

This fix leaves the algorithm used in the past untouched, but adds a
new rule. When a new or old master which is empty and without slaves,
are assigend with their first slot, if other masters in the cluster have
slaves, they are automatically considered to be targets for replicas
migration.