bs-thread-pool release 5.1.0-2 for unstable

[dgit distro=debian split --quilt=gbp]
[dgit please-upload source=bs-thread-pool version=5.1.0-2 upstream-tag=upstream/5.1.0 upstream=0165c2f153ae8157fcb50f637d89f9d2fccbb5e1]