Hide mock interfaces from users

[Neo2308]

• Moved mocks in /lib to /lib/internal to prevent getting imported by others
• Moved mocks in /store to _test.go to prevent getting imported by others
• (Housekeeping) Removed mockgen commands for memcache & ristretto since they have been moved to mockgen
• Renamed import to resolve warnings

[Neo2308]

@eko This resolves part of #282 about exposing the mocks as public API.
Based on my understanding, the library should only be included in the binary if its used and after this PR:

• For /store:
  • mocks are in internal folder so can't be used directly by users
  • mocks package is only used in test files by the library hence should not appear in the binary
• For /lib:
  • mocks are in _test.go files so can't be used directly by users nor should they appear in the binary

Haven't tested whether they will show up in the binary or not (just speculation, can verify separately later), created this because atleast they will not show up on the public API.