Add new mutex strategies

[jwoodrow]

Add new strategies to RedisMutex

• concurrent: Prevent concurrent executions of the block provided, once the block ends, release of the mutex "lock" juste like "standard"
  • When you think about it, the "standard" implementation of redis-mutex is a concurrency restriction of 1
• cumulative: Prevent a block from being executed too many times over a slidingperiod of time (sliding window looking back expire seconds)
• windowed: Prevent a block from being executed too many times over a fixed period of time (at the end of expire seconds the execution counter would get reset to 0

Try to make everything backwards compatible by making concurrent strategy and limit 1 the default for the 2 new options introduced

• limit: How many concurrent block executions do you want to limit yourself to (default 1)
• type: The type of mutex strategy you wish to use (default concurrent [cumulative, windowed])

Also for easier contribution:

• Added github action matrix workflow to run CI (github actions is free for public repositories so might as well use it)
• Added a Dockerfile to run dev containers for each ruby version you might have issues in the CI with (I know I did)
• In CI, just use the redis that's provided by env variables instead of going for 15 all the time

Still need to add some concurrent/cumulative/windowed specific specs but this currently makes it so the specs work without the need to update them

---

Here are some simple testing scripts to see how each of these strategies work (adding them as I go)

concurrent + limit > 1

require 'redis-mutex'
RedisClassy.redis = Redis.new

RedisMutex.with_lock('my_lock', limit: 2, block: 2) do
  begin
    puts 'I have the lock! 1'
    RedisMutex.with_lock('my_lock', limit: 2, block: 2) do
      begin
        puts 'I have the lock! 2'
        RedisMutex.with_lock('my_lock', limit: 2, block: 2) do
          begin
            puts 'I have the lock! 3'
          rescue RedisMutex::LockError
            puts 'I could not get the lock 3'
          end
        end
      rescue RedisMutex::LockError
        puts 'I could not get the lock 2'
      end
    end
  rescue RedisMutex::LockError
    puts 'I could not get the lock 1'
  end
end
# => I have the lock! 1
# => I have the lock! 2
# ... 2 seconds pass
# => I could not get the lock 3