Context keys must have a unique type in order to prevent
any unintented clashes. The code used int(1) as key.
Fix it by implementing the pattern recommended by package context.
The test chain generated by makeChainFork included invalid uncle
headers, crashing the generator during the state commit.
The headers were invalid because they used the iteration counter as the
block number, even though makeChainFork uses a block with number > 0 as
the parent. Fix this by introducing BlockGen.Number, which allows
accessing the actual number of the block being generated.
State and receipt deliveries from a previous eth/62+ sync can hang if
the downloader has moved on to syncing with eth/61. Fix this by also
draining the eth/63 channels while waiting for eth/61 data.
A nicer solution would be to take care of the channels in a central
place, but that would involve a major rewrite.
Unexpected deliveries could block indefinitely if they arrived at the
right time. The fix is to ensure that the cancellation channel is
always closed when the sync ends, unblocking any deliveries. Also remove
the atomic check for whether a sync is currently running because it
doesn't help and can be misleading.
Cancelling always seems to break the tests though. The downloader
spawned d.process whenever new data arrived, making it somewhat hard to
track when block processing was actually done. Fix this by running
d.process in a dedicated goroutine that is tied to the lifecycle of the
sync. d.process gets notified of new work by the queue instead of being
invoked all the time. This removes a ton of weird workaround code,
including a hairy use of atomic CAS.