BUG/MINOR: ssl/threads: Make management of the TLS ticket keys files thread-safe

A TLS ticket keys file can be updated on the CLI and used in same time. So we
need to protect it to be sure all accesses are thread-safe. Because updates are
infrequent, a R/W lock has been used.

This patch must be backported in 1.8
diff --git a/include/common/hathreads.h b/include/common/hathreads.h
index 143cf2c..30009cc 100644
--- a/include/common/hathreads.h
+++ b/include/common/hathreads.h
@@ -291,6 +291,7 @@
 	EMAIL_ALERTS_LOCK,
 	PIPES_LOCK,
 	START_LOCK,
+	TLSKEYS_REF_LOCK,
 	LOCK_LABELS
 };
 struct lock_stat {
@@ -407,6 +408,7 @@
 	case EMAIL_ALERTS_LOCK:    return "EMAIL_ALERTS";
 	case PIPES_LOCK:           return "PIPES";
 	case START_LOCK:           return "START";
+	case TLSKEYS_REF_LOCK:     return "TLSKEYS_REF";
 	case LOCK_LABELS:          break; /* keep compiler happy */
 	};
 	/* only way to come here is consecutive to an internal bug */