proxy: Add a 30-second timeout for requests in-flight to upstream

It's a little more complicated than that, actually. For the various
states that involve reading from, or writing to, the upstream fd,
if the amount of time spent in that state is > 30 seconds, we reconnect
to the server and resend the request.

we also introduce a 15-second reconnect dampener to keep us from stressing
things unduly. This may need to be decreased, or turned into an exponential
backoff, at some point.

src/proxy.h

@@ -14,6 +14,12 @@
   #include "prefetch.h"
 #endif
 
+/** UPSTREAM_TIMEOUT
+ * How long ( in ms ) to allow for upstream to respond. If it takes longer
+ * than this, we will cancel the current request-response to them and resubmit
+ */
+#define UPSTREAM_TIMEOUT 30 * 1000
+
 struct proxier {
 	/* The flexnbd wrapper this proxier is attached to */
 	struct flexnbd*   flexnbd;