Request next headers before processing current ones to speed up header sync.

[QuantumExplorer]

Currently header sync is a bit slower than optimal, ideally it should happen in 2 (or more) threads. One that constantly gets headers and puts them in a buffer, and other than processes them. This should speed up header sync.

[PastaPastaPasta]

TODO; bench how long processing a batch of 2k headers takes

[xdustinface]

Has been improved in #411