Add bandwidth limitations to benchmarks

[hannahhoward]

Currently, benchmarks in dup_blocks_test.go (perhaps this should just become benchmarks.go) use testnet, which simply assigns a latency to each request for a block. This means you can in theory request an infinite amount of blocks from a single peer, they will only each be delayed by that peer's latency. In reality, presumably you would eventually fill the pipe in terms of bandwidth, causing additional delays. Moreover, your own peer might have a maximum bandwidth no matter whom it requests blocks from. We should add these facilities to testnet, and then write benchmarks that use them, to provide more accurate real world benchmarks for bitswap.