Update src/proto/nc_redis.c
Co-authored-by: NTyson Andre <tyson.andre@uwaterloo.ca>

And update documentation

Revert the addition of blocking commands - making them a compilation
option is probably a poor way to distribute it.

Support `COMMAND` (used by redis-cli) and a placeholder no-argument command
forwarded to a random server.

This amends the approach originally proposed in
https://github.com/twitter/twemproxy/pull/565

We don't need to keep a stack, we only need to keep track
of how many more remaining elements need to be parsed from the redis
response, decreasing it when an element is processed,
and increasing it when a new redis array is seen based on the array length

i.e. the response of georadiuswithcoord.
This is achieved by introducing a small stack in struct msg,
this stack remembers the number of arguments to be parsed of current multibulk
and all nesting multibulks.
Co-Authored-By: Nqingping209 <qiw209@gmail.com>
Co-Authored-By: NTyson Andre <tysonandre775@hotmail.com>

This is a subset of https://github.com/twitter/twemproxy/pull/565
(only the multibulk implementation changes by qingping209)

nutredis currently only supports keys for sharding in the first position.
That can be fixed but using `operation` is wrong

Leave more complicated implementations for subsequent PRs

If this is approved, documentation should also be added to
notes/redis.md

Some commands such as EXISTS have been changed to support multiple
arguments.

Copied from https://github.com/ifwe/twemproxy/pull/32/files
which also updates unit tests (depends on changes to the test framework)
- That PR has the script used to extract updated argument counts from
  redis-doc's commands.json file
- See https://github.com/twitter/twemproxy/pull/598 for fixing tests

Related to https://github.com/twitter/twemproxy/pull/589/files

And add unit tests of redis messages being parsed successfully

Fragmentation would likely be slower.
It would involve splitting up the request into multiple smaller
requests (allocating and zeroing out an array)
and concatenating strings to rebuild the original request.

Fragmentation is also more likely to be prone to edge cases such as
those mentioned in https://github.com/twitter/twemproxy/pull/597
and https://github.com/twitter/twemproxy/pull/595

ncontinuum is `160 * the number of **non-ejected** hosts` for the ketama
distribution, where 160 is the ketama consistent hashing points per server.
However, the index returned by msg_backend_idx is a number between
0..nservers-1.

- This is a waste of time zeroing out 160 times as much memory as needed
- When there are 0 servers, this previously malloc()ed a pointer of size 0 bytes
  and nutcracker would write to invalid memory.

Also, update redis nosetest expectation because newer redis versions changed the
error message to mention replica instead.

Fixes https://github.com/twitter/twemproxy/issues/563 for redis and memcached.
See the discussion there for more details.

refactored how redis_auth is handled through authenticated option; redis_auth is only enabled for a redis pool

- added a callback that can inspect a req/rsp before it is swallowed
- this callback is now used with redis so it can check on failed selects