webdav: Make threading and TCP connection limits configurable

With the increase use of our webdav door, we see a need to tune
certain aspects of the webdav door that are currently hardcoded.

This patch exposes those aspects to the configuration.

Since I suspect that other sites will run into similar problems,
I suggest merging this into dCache 2.10.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: yes
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: https://rb.dcache.org/r/7173/