Fixes for scan/sscan/zscan/hscan & transaction/multi handling

[Xon]

• Fixes type hinting on for scan/sscan/zscan/hscan
• Aligns behavior of scan/sscan/zscan/hscan with phpredis of how an empty/falsy iterator is handled.
• Fix errors in exec() could cause the redis connection and credis internal state to become out of sync
• Prevent scan/sscan/zscan/hscan from being used in pipeline/multi mode, as phpredis would cause trigger a fatal php error instead of throwing an exception in this case
• Fix calling mget([]) in standalone mode didn't match phpredis mode
• Fix multi mode failing when commands where issued between multi() and pipeline(). Recommended usage is however pipeline()-> multi()->...->exec()

This breaks pipeline mode for these functions as they return early, but mget was already doing this. Documented in #182

[colinmollenhour]

@Xon Thanks for the PR. I'm not sure about it though since it would break code that uses chaining, or is it just never practical to use these methods with chaining?

[Xon]

Using this particular methods in chaining mode is fairly difficult as you need to returned iterator token for the next call, but you could batch them with other methods I guess.

I was planning on adding a PR after this to patch these methods so pipeline mode could push an early result into the command array so the methods are never sent to redis. This would fix the compatibility issues with pipelining

[Xon]

Redis::scan(): Can't call SCAN commands in multi or pipeline mode!

So it looks like phpredis just doesn't allow scan in pipeline/multi mode, and a mget([]) is silently dropped from the exec() results.

I'll add some test to catch these

[Xon]

After a bit more testing, using the scan-related commands never worked in pipeline/multi mode with standalone credis mode. The iterator was never updated as expected

[Xon]

Forcing the connection closed and resetting pipelining/transaction state ended up being required to actually debug why unrelated tests where failing. redis/credis getting out of sync on the protocol level is yuck :(

This ended up being a lot more involve but fixes #182 so the same behavior is observed between phpredis and standalone mode when in transaction/pipeline mode. In previous cases calling these functions in standalone mode would either error or be unusable.

I've updated the PR with a description of the various bugs fixed. Most of them are actually relatively obscure that wouldn't be hit in normal usage. Basically pipeline()->multi() should be the strongly recommended pattern when interacting with the multi() command.

[colinmollenhour]

Nice work @Xon!