Fix/23 change mutex lock conditionally

[Crambor]

Mostly minor changes here - just #23 (unique_lock replaces the original lockguard) and #24 (added a conditional for error checking)

Most of the line changes are whitespace, because I brought in the original lib/umbridge.h into the hpc directory. This has also therefore inherited the lastest commits, such as the additional logging.

[linusseelinger]

I think we should merge all changes into the main lib/umbridge.h and get rid of the copy in hpc/. The only reason the copy in hpc/ is around is because it holds the disabled mutex which we need for the load balancer. Could you apply your changes to the main library and remove the hpc copy?

[linusseelinger]

Looks good so far! Going forward, I think we should either

• only modify the hpc/lib/ version here and proceed with merging in a separate PR
• or fully merge with lib/, completely eliminating hpc/lib/ and adapting the load balancer appropriately