Allow setting maxOustanding and maxTCPClientThreads in configuration

Allow setting maxOustanding and maxTCPClientThreads in configuration

This commit adds the setMaxTCPClientThreads() and
setMaxUDPOutstanding() directives.
These controls, respectively, the maximum number of TCP threads
handling client connections and the maximum number of oustanding
UDP queries to a given backend server.
setMaxUDPOutstanding() is only usable at configuration-time, and
not at runtime.