Make HttpClientManager threadsafe

This allows a single HttpClientManager to be used from more than one
thread at the same time. We discussed having one HttpClientManager
per job queue thread. Assuming there should be one HTTP thread per
node, this would mean quadratic growth with the number of nodes. By
being able to reuse the manager, this problem is a defused a bit.

This patch also fixes a typo in HttpClientRequestExecutor.

Reviewed-by: iustinp