AsyncGetBlocks take a set of block cids, a pubsub channel for incoming blocks, a want function, and a close function, and returns a channel of incoming blocks.

SyncGetBlock takes a block cid and an async function for getting several blocks that returns a channel, and uses that function to return the block syncronously.

GetBlocksFunc is any function that can take an array of CIDs and return a channel of incoming blocks.

WantFunc is any function that can express a want for set of blocks.