Introduce DistributedMutex class

[mvorisek]

Originally, RedisMutex supported multiple clients, but there is no reason to not support any mutex as a distributed backend.

BC break: RedisMutex constructor newly accepts only 1 Redis client

For one 1 Redis client: Pass the client directly to the RedisMutex constructor.

For multiple Redis clients: Create RedisMutex instance for each client and pass them into DistributedMutex constructor.