Allow to throttle the number of blocks read per second from a filesource

[fschoell]

To be able to better control the usage on lower tiers it would be helpful to throttle the number of blocks per second that get fed into the Substream workers or send out from Firehose.

The dauth plugin could then control it from a trusted header like X-SF-THROTTLE-BLOCKS, which gets parsed in the firehose/substream app and then passed on as parameter or option to bstream when creating a new filesource.

Not sure where the best way would be to implement such a throttle, maybe we could just add a sleep here