refactor(bs/decision.Engine): pass in Entry

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>
fix: add lock to taskQueue

@whyrusleeping may wanna have a look and make sure i didn't screw
anything up here

BenchmarkInstantaneousAddCat1MB-4            200          10763761 ns/op
97.42 MB/s
BenchmarkInstantaneousAddCat2MB-4       panic: runtime error: invalid
memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0xbedd]

goroutine 14297 [running]:
github.com/jbenet/go-ipfs/exchange/bitswap/decision.(*taskQueue).Remove(0xc2087553a0,
        0xc2085ef200, 0x22, 0x56f570, 0xc208367a40)
    /Users/btc/go/src/github.com/jbenet/go-ipfs/exchange/bitswap/decision/taskqueue.go:66
    +0x82
github.com/jbenet/go-ipfs/exchange/bitswap/decision.(*Engine).MessageSent(0xc20871b5c0,
        0x56f570, 0xc208367a40, 0x570040, 0xc208753d40, 0x0, 0x0)
    /Users/btc/go/src/github.com/jbenet/go-ipfs/exchange/bitswap/decision/engine.go:177
    +0x29e
github.com/jbenet/go-ipfs/exchange/bitswap.(*bitswap).send(0xc20871b7a0,
        0x56f4d8, 0xc208379800, 0x56f570, 0xc208367a40,
        0x570040, 0xc208753d40, 0x0, 0x0)
    /Users/btc/go/src/github.com/jbenet/go-ipfs/exchange/bitswap/bitswap.go:352
    +0x11c
github.com/jbenet/go-ipfs/exchange/bitswap.(*bitswap).taskWorker(0xc20871b7a0,
        0x56f4d8, 0xc208379800)
    /Users/btc/go/src/github.com/jbenet/go-ipfs/exchange/bitswap/bitswap.go:238
    +0x165
    created by
    github.com/jbenet/go-ipfs/exchange/bitswap.New
    /Users/btc/go/src/github.com/jbenet/go-ipfs/exchange/bitswap/bitswap.go:66
    +0x49e
doc: some comments about the future of the decision engine

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>