Use separate IO context for socket IO (#168)

* Keep track of number of requests currently being processed
* Reject new requests when number of in flight requests exceeds a
  configurable limit
* Track time spent between request arrival and start of request
  processing

Signed-off-by: CJ Cobb <ccobb@ripple.com>

Co-authored-by: natenichols <natenichols@cox.net>

src/rpc/WorkQueue.cpp

@@ -0,0 +1,11 @@
+#include <rpc/WorkQueue.h>
+
+WorkQueue::WorkQueue(std::uint32_t numWorkers, uint32_t maxSize)
+{
+    if (maxSize != 0)
+        maxSize_ = maxSize;
+    while (--numWorkers)
+    {
+        threads_.emplace_back([this] { ioc_.run(); });
+    }
+}