Remove request allocation backpressure

[hannahhoward]

What

Request allocation backpressure was thrown in quite ad-hoc to respond to miner concerns about memory usage. (as opposed to response allocation backpressure which has seen pretty careful development over a while).

I have been aware of potential issues with it for some time, but have failed to properly address them as they require a fair amount of plumbing to get right.

Given the presences of outgoing request limits, it seems prudent to remove allocation backpressure on the requestor side as a first step.

How

• Remove top level options for configuring allocation backpressure on the Requestor side
• Remove allocation backpressure from impl graphsync sync receiver
• Remove allocation backpressure calls from RequestManager
• Remove allocation backpressure calls from AsyncLoader
• Remove allocation backpressure test for impl

[hannahhoward]

Follow-up: at this point, we have simultaneous outgoing requests limits, where we didn't before. The memory backpressure on the requestor side has been a source of issues and was put in somewhat haphazardly at a time when we didn't have these -- is it neccesary? should it be removed entirely?

[rvagg]

I believe this was introduced primarily in https://github.com/ipfs/go-graphsync/pull/205/commits/9171ce6bccd62bf4303c12edd07e278713e6da06