Only 1 block synced at a time

[SurfingNerd]

During a Stress test at the end of the 0.10 lifecycle, we encountered a problem with blocks that are filled > 99%.
The Nodes encountered network timeouts, therefore we reduced the number of blocks that are requested at maximum to 1.

A more elegant solution was already discussed in this issue: #209

[SurfingNerd]

This change has not shown performance problems yet, we are still avaluating if we should stay at 1 block max, or increase that value again.

[SurfingNerd]

current payload limits:
Max Payload: 64 MB
Max Payload Soft (per sync cycle): 4 MB