tcache: producer floors and consumer tails

[vladimir-ea]

This adds a mechanism for consumers to accurately set their tail value without skipping TCacheReads that arrive out of order:

• every TCacheRead now includes an emitter id and a floor value
• every producer also periodically publishes their floor value in TCacheHead
• for tiles that read, store and forward TCacheReads, the floor is their own consumer tail.

the value of floor means: I will not produce an event containing a TCacheRead with a seq lower than floor.

• readers track the received floors per TCache per emitting Tile (emitters); combined with the locally acquired reads this is used to calculate the tail
• idle consumers read the producer's published floor, so they follow the producer

This mechanism has the following consequences:

• all TCacheReads that are held or stored across tile loops must be acquired
• readers declare expected emitters when constructing consumers (in Tile::try_init())
• producers call .loop_start() at the start of every tile loop - this fixes the producer's floor for the current loop