Process received blocks in separate channel

Now that the database is threadsafe, the blocks received over the network can be processed in a separate goroutine.

Currently block processing occurs in the same thread as the network loop. If a block processing queue is used in a separate goroutine, the network could receive blocks faster than it can process them, so the queue might fill up. This case should be analyzed and handled properly.

Trace the network loop and the current block-processing path mentioned in the issue. Analyze how a separate processing queue should behave when network reception outpaces processing, and consider the work complete when processing is separated and queue saturation is handled safely.

Written by the indexing model from the issue text.