This is an attempt to fix #281.
This PR was also tested using the test suite of our service.

There are a few decisions I made regarding this PR, please let me know
if it's not appropriate:

1. Error from Redis now contains the redis connection URL
- To fix this, I created `MockRedis::Error` module which decorate
`Redis::CommandError` and related errors with the connection URL
- I replace most of the `Redis::CommandError` with the new module
wrapper
2. Some methods now return integer as output instead of boolean and
`exists_returns_integer` is removed
  - I replace the output as is
3. Interfaces of most data structures (Hash, Set, ...) now validate type
to be 4 main primitive types
- `assert_type` is added to validate the arguments. Currently it doesn't
look very clean, so any improvements would be great.
4. `sadd`, `srem` now support multiple arguments
5. Transaction: `multi` now needs to call with a block and
`discard`/`exec` cannot be called outside that block anymore
- I tried to refactor the logic to make the tests pass, but I'm not
entirely confident that it works correctly

Currently failing due to needing redis-rb v5 (see also #281 and #293)

Fix #318