[rqd] Add frame recovery logic for docker mode

Whenever rqd restarts it loses track of all the frames launched by it that haven't
finished. This change adds a new configurable option to backup frame states to a file,
that is used to recover the frame cache state and try to re-bind to the running frames.

This first version only works on docker mode

proto/rqd.proto

@@ -115,6 +115,7 @@ message RunFrame {
     int32 num_gpus = 23;
     report.ChildrenProcStats children = 24;
     string os = 25;
+    int32 pid = 26;
 }
 
 message RunFrameSeq {

rqd/rqd/rqconstants.py

@@ -162,6 +162,11 @@
 DOCKER_MOUNTS = []
 DOCKER_SHELL_PATH = "/bin/sh"
 
+# Backup running frames cache. Backup cache is turned off if this path is set to
+# None or ""
+BACKUP_CACHE_PATH = "/tmp/opencue/running_frames_backup.dat"
+BACKUP_CACHE_TIME_TO_LIVE_SECONDS = 60
+
 try:
     if os.path.isfile(CONFIG_FILE):
         # Hostname can come from here: rqutil.getHostname()