Allow to execute multiple cache sets in a single pipeline

Looking at profiling data of a cold cache bootstrap, I can see that 208 setMultiple() calls are expanded to 1209 set() calls (that's on 11.x with #3537962: EntityFieldManager::getFieldDefinitions() per-bundle caching can be expensive.

That's 160ms in total. However, only 26% of that time (41ms) is actually PhpRedis::exec(), the other 60% are serializing the data and compressing it. 41ms is around 1.7% of the entire request time (2.3s). It might be a higher percentage without blackfire overhead, it's specifically around 30ms IO wait, but still, not a lot to gain.

Yes, I am using igbinary, or at least I thought it was, had a configuration error (#3570217: Update docs to clarify that igbinary serializer override must be done after services.redis.yml include), I fixed that yesterday and confirmed that saves around 8% of space in redis after compression (so comparing serialize + compression vs igbinary + compression). Measured with my updated drush redis report. Didn't manage to reliably compare seralize and unserialize speeds, variation is pretty high. It helps, but it's not huge.

I've also done various benchmarks in #3014514: Make igbinary the default serializer, if available some time ago.

This was trickier than expected, but I'm pretty confident in our test coverage, which managed to uncover a weird race condition of a pipeline happening within a pipeline which failed hard.