add user-client mapping.

[ztxiuyuan]

As you can see in redis/redis#12245 ,user-client mapping has been added to the server to facilitate statistics on the usage of each user connection.

client list user return specified user connection info

127.0.0.1:6379> CLIENT list user doug
id=5 addr=127.0.0.1:59469 laddr=127.0.0.1:6379 fd=9 name= age=33 idle=28 flags=N db=0 sub=0 psub=0 ssub=0 multi=-1 watch=0 qbuf=0 qbuf-free=0 argv-mem=0 multi-mem=0 rbs=1024 rbp=0 obl=0 oll=0 omem=0 tot-mem=1824 events=r cmd=auth user=doug redir=-1 resp=2 lib-name= lib-ver=
id=6 addr=127.0.0.1:59490 laddr=127.0.0.1:6379 fd=10 name= age=19 idle=15 flags=N db=0 sub=0 psub=0 ssub=0 multi=-1 watch=0 qbuf=0 qbuf-free=0 argv-mem=0 multi-mem=0 rbs=1024 rbp=0 obl=0 oll=0 omem=0 tot-mem=1824 events=r cmd=auth user=doug redir=-1 resp=2 lib-name= lib-ver=

client count return specified user connetcion count

127.0.0.1:6379> CLIENT COUNT doug
"2"

[codecov]

Codecov Report

Attention: Patch coverage is 50.63291% with 39 lines in your changes are missing coverage. Please review.

Project coverage is 68.50%. Comparing base (93f8a19) to head (689db89).
Report is 8 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #405      +/-   ##
============================================
+ Coverage     68.43%   68.50%   +0.06%     
============================================
  Files           109      109              
  Lines         61681    61760      +79     
============================================
+ Hits          42214    42309      +95     
+ Misses        19467    19451      -16     

Files	Coverage Δ
src/acl.c	89.02% <100.00%> (+0.04%)	⬆️
src/commands.def	100.00% <ø> (ø)
src/networking.c	83.84% <46.57%> (-1.24%)	⬇️

... and 10 files with indirect coverage changes

[zuiderkwast]

Hi! Thanks!

I'm just looking at the commands and arguments now, not the code yet. This PR adds two different things.

• CLIENT LIST: Adding a filter argument USER username is a good idea. But ID id-1 id-2 ... must be the last argument because it accepts multiple arguments.

• CLIENT COUNT: It is too specific to only user names. It needs to be able to use the same filters as CLIENT LIST, for example

  CLIENT COUNT TYPE normal USER doug ID 5 7 8 9
  

  The return value should be an integer, not a string.

• CLIENT KILL has the same arguments as CLIENT LIST. It needs to support USER doug too.

Please read this proposal: redis/redis#12311 -- we don't need to implement all of it but we should follow the logic explained there.