eth, p2p: improve write timeouts and behaviour under load

Fixes #1206

The previous value of 5 seconds causes timeouts for legitimate messages
if large messages are sent.

The protocol tests were commented out when eth/downloader was introduced.

Nodes that are out of sync will queue many transactions, which causes
the initial transactions message to grow very large. Larger transactions
messages can make communication impossible if the message is too big to
send. Big transactions messages also exhaust egress bandwidth, which
degrades other peer connections.

The new approach to combat these issues is to send transactions in
smaller batches. This commit introduces a new goroutine that handles
delivery of all initial transaction transfers. Size-limited packs of
transactions are sent to one peer at a time, conserving precious egress
bandwidth.

If blocks get larger, sending 256 at once can make messages large
enough to exceed the low-level write timeout.

jsre: patch up the pretty printer to have a decent look

Precompiled ec recover padding

Fix various data races in eth and core

Fix RPC sign

Differentiate between 0 and unspecified gas/gasprice

core: skip genesis block for reprocess. Closes #1202

eth: implement the NewBlockHashes protocol proposal

eth/downloader: gather and ban hashes from invalid chains