Fix redis cache

[dexterlb]

Since commit deeeaa3 the redis_cache module seems to have been broken in multiple places:

The format of timestamp keys is malformed

This is because of an apparent attempt to refactor code to be compatible with older versions of python: the refactored code contains a silly copy-paste error that results in curly braces appearing in the keys, which breaks timestamp handling.

This PR fixes this mistake.

Listing child banks is broken, which results in commands like salt-run cache.list minions returning nothing every time

Child banks should be listed with _get_bank_redis_key instead of _get_bank_keys_redis_key

This PR fixes this mistake.

New objects get inserted in the wrong paths

The function _build_bank_hier seems to have been refactored from using a for-loop and several accumulating variables into a more functional style with itertools.accumulate. However, instead of adding the names of child banks as set-members into the parent bank, the refactored code only adds a "." member into each newly-created bank. This is a bug and causes all queries for nested banks to fail.

This PR rewrites the function in a way that:

• Retains the functional style established by commit 9233e1c
• Reimplements the old behaviour of inserting the names of child banks into parent banks
• Retains the logic that adds a "." member into newly-created banks

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[twangboy]

This looks great. Could we get some tests and a changelog, please?