Reduced competition during parallel operations and removed the num-writers flag

The latter isn't really helping parallelism as long as we are not caching anything,
as it will be as slow as the slowest node in the pipeline.
Initial implementation of parallel writing

It builds, but still has to be verified and tested
Fixed windows build and ruled out a possible deadlock.
First version which parallelizes writes without hanging.
First parallel writer test seems to do something now
sealed-copy logging now writes proper log messages.

Optimized data structures - BasicResult is not stored as pointer anymore
Readers now support multiple devices

That way, even if there is just one stream set, the system will read
from two steams in parallel.
Initial implementation of --verify, which doesn't really work

Design-wise, the implementation is not right in many ways