Helper queue for multi-input elements

[mat-hek]

The idea is to have a queue that multi-input pads could use to consume buffers at proper speed. The goal is to make mixers and muxers code simpler, more declarative and thus less error-prone.

The queue should:

• Consume buffers at equal speed from all pads, based on timestamps
• Return pause_auto_demand and resume_auto_demand actions to avoid keeping too many buffers
• Handle dynamically starting and ending streams

To verify the queue's functionality, we should use it in at least one muxer, for example in https://github.com/membraneframework/membrane_flv_plugin and mixer, for example in https://github.com/membraneframework/membrane_audio_mix_plugin

[FelonEkonom]

Done:

• basic functionalities (push/pop)
• waiting on pad
• specify average chunk size, expresesed in time duration
• specify pause demand boundary in time duration
• flushing queue
• specify timestamps offsets explicitly / allow different synchronization strategies

Issues:

• lack of callback context

TODO:

• write benchmarks and optimize the queue
• live queue

Live queue concepts:

• need of having master stream or some other way of telling the live queue what is the current live time
• live queue should return buffers according to the clock
• outdated buffers should be distincted from actual buffers