I initially made the client block if the 100-element buffer was
exceeded. It turns out that this is inconvenient for simple uses of the
client which subscribe and perform calls on the same goroutine, e.g.

    client, _ := rpc.Dial(...)
    ch := make(chan int) // note: no buffer
    sub, _ := client.EthSubscribe(ch, "something")
    for event := range ch {
        client.Call(...)
    }

This innocent looking code will lock up if the server suddenly decides
to send 2000 notifications. In this case, the client's main loop won't
accept the call because it is trying to deliver a notification to ch.

The issue is kind of hard to explain in the docs and few people will
actually read them. Buffering is the simple option and works with close
to no overhead for subscribers that always listen.

eth/downloader: fix the stall checks/drops during sync

rpc: add new client, use it everywhere

eth, eth/downloader, eth/fetcher: delete eth/61 sync code

The new client implementation supports concurrent requests,
subscriptions and replaces the various ad hoc RPC clients
throughout go-ethereum.

If a batch request contained an invalid method, the server would reply
with a non-batch error response. Fix this by tracking an error for each
batch element.

The server delayed closing of connections for 3s when stopping. This was
supposed to allow for slow handlers, but it didn't really work. When
geth quits, it will just exit immediately after quitting the server.

Removing the timer makes testing easier because all connections will be
closed after Stop returns.

eth/api: rename signAndSendTransaction to sendTransaction

The eth/61 protocol was disabled in #2776, this commit removes its
message handlers and hash-chain sync logic.

containers/docker: update to alpine 3.4

eth: cancel DAO challenge on peer drop (annoying log)

Godeps: bump github.com/syndtr/goleveldb/... to ab8b5dcf104

Fix #2710 Filter race: concurrent map read and map write

cmd/bootnode: Add support for outputting a node's ID on the command line

eth: fixed chaindb upgrade

cmd, core, eth, miner, params, tests: finalize the DAO fork

circleci: enable docker based hive testing

vm: Replace some SstoreClearGas with SstoreResetGas

Godeps, rpc: switch back to package npipe

This adds compatibility with Go 1.7, where context has
moved into the standard library.

The named pipe implementation from go-winio has some issues
that need to be addressed before we can use it again.