First commit of redis cache utilising predis

[tinoest]

@Spuds not sure if you want this as part of the core, or if I am doing this the right way.

The ip for the redis server is hardcoded currently, so if you do a checkout edit this line to what your ip is to get it working.

$this->obj = new \Predis\Client('tcp://172.19.0.5:6379');

I will change and work out how to add multiple different servers if you are happy using predis. It seems to work ok otherwise.

[codecov]

Codecov Report

Patch coverage: 53.33% and project coverage change: +0.17 🎉

Comparison is base (c5730e1) 30.45% compared to head (fa128c0) 30.62%.

Additional details and impacted files

@@                Coverage Diff                @@
##             development    #3708      +/-   ##
=================================================
+ Coverage          30.45%   30.62%   +0.17%     
- Complexity         16416    16445      +29     
=================================================
  Files                429      430       +1     
  Lines              84387    84372      -15     
=================================================
+ Hits               25703    25843     +140     
+ Misses             58684    58529     -155     

Impacted Files	Coverage Δ
...rces/ElkArte/AdminController/ManageAttachments.php	8.74% <ø> (ø)
sources/ElkArte/AdminController/ManageFeatures.php	18.25% <0.00%> (ø)
sources/ElkArte/AttachmentsDirectory.php	9.90% <ø> (ø)
sources/ElkArte/Controller/Attachment.php	9.58% <0.00%> (ø)
sources/ElkArte/Controller/Groups.php	29.48% <0.00%> (ø)
sources/ElkArte/Controller/ModerateAttachments.php	0.00% <0.00%> (ø)
sources/ElkArte/Controller/Stats.php	0.00% <0.00%> (ø)
sources/ElkArte/Modules/Attachments/Display.php	100.00% <ø> (ø)
sources/ElkArte/Modules/Random/Display.php	83.33% <ø> (ø)
sources/ElkArte/SettingsForm/SettingsForm.php	100.00% <ø> (ø)
... and 16 more

... and 11 files with indirect coverage changes

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