Added support for HierarchicalPoolInterface

[Nyholm]

Adding support for hierarchy. See Readme at: https://github.com/php-cache/hierarchical-cache

Related PRs:

• Added a trait to use with adapters hierarchical-cache#2
• Hierarchy support taggable-cache#14
• Added integration tests for hierarchy adapters integration-tests#23
• Bumped versions of taggable adapter-common#6

[Nyholm]

Ping @magnusnordlander and @nicholasruunu. It is a lot of PRs but have a look at this one and the one at hierarchical cache repo.

[codecov-io]

Current coverage is 100.00%

Merging #7 into master will not affect coverage as of 5a8e30f

@@            master      #7   diff @@
======================================
  Files            1       1       
  Stmts           19      20     +1
  Branches         0       0       
  Methods          6       6       
======================================
+ Hit             19      20     +1
  Partial          0       0       
  Missed           0       0       

Review entire Coverage Diff as of 5a8e30f

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[Nyholm]

Ping @aequasi this is now ready for a review

[Nyholm]

Awesome. Thank you