Unit tests and bug fixes for XmlRpcpp

[trainman419]

The initial motivation for this was to fix some rare crashes that we were seeing in xmlrpcpp where it would leak file descriptors (as described in https://answers.ros.org/question/250393/rosout-leaks-file-descriptors/ ) and then would crash if given a socket file descriptor with descriptor number above FD_SETSIZE (commonly 1024)

This also fixes #914, where an incoming XmlRpc request would cause the server to use 100% of a core if it could not successfully call accept(). (replaces #977, #960). There are two fallback mechanisms here:
Measuring the number of open file descriptors and close new incoming connections if the number of free file descriptors falls below a threshold.
If accept fails completely, stop monitoring the server socket for 1 second and then re-add it, so that we can still accept new connections if the old connections eventually close.

• Fix for file descriptor leaks in XmlRpcClient.
• Reject incoming connections in XmlRpcServer to prevent file descriptor exhaustion.
• Timed retry if accept() fails.
• Fix handling of partial buffer reads and writes in XmlRpcClient.
• Fix use of uninitialized memory in XmlRpcSocket::get_port.
• Break dependency on XmlRpc.h header to make targeted mocking easier.
• Better error reporting when address lookup fails. (Add error message when topic can't connect (roscpp and rospy) #222)
• Report symbolic state names in XmlRpcClient.
• Make async XmlRpcClient terminate correctly on error.
• Update comments to reflect the fact that an XmlRpcDispatch "exception" actually indicates out-of-band TCP data has arrived.
• Fix XmlRpcDispatch to correctly honor the Exception request again.
• Make XmlRpcUtil::error go to stdout to match log and provide better error reporting. (xmlrpc should print errors #364)
• Lots of unit tests.

cc @kmactavish @mikepurvis @mgrrx @ruffsl

[dirk-thomas]

The size of the diff is pretty big which makes it rather difficult to review. Can the individual fixes and improvements be split into separate pull requests or at least separate commits?

[trainman419]

Ok; I'll try to split this into separate pull requests.

[trainman419]

First part of this: #1216 . Many of the other tests depend on that header refactoring, so that will need to merge before I can submit the remainder of my tests and fixes as smaller PRs.

[trainman419]

Second part: #1218

[trainman419]

Third part: #1221

[trainman419]

Fourth part: #1232

[trainman419]

Part 5: #1242

[trainman419]

Part 6: #1243

[trainman419]

Part 7: #1244