add queue settings

The patch added the ability to set the queue settings(by calling
"queue.set_settings(opts)").
Now only one setting is available - "ttr"(time to release).
"ttr" in seconds - the time after which, if there is no active
connection in the session, it will be released with all its tasks.

Part of #85

Author: LeonidVas

queue/abstract.lua

@@ -28,7 +28,8 @@ local queue = {
             return not (self[tube_name] == nil)
         end
     }),
-    stat = {}
+    stat = {},
+    settings = {}
 }
 local MAX_TIMEOUT      = 365 * 86400 * 100       -- MAX_TIMEOUT == 100 years
 local TIMEOUT_INFINITY = 18446744073709551615ULL -- Set to TIMEOUT_INFINITY
@@ -532,6 +533,22 @@ function method.create_tube(tube_name, tube_type, opts)
     return self
 end
 
+function method.set_settings(opts)
+    opts = opts or {}
+
+    for key, val in pairs(opts) do
+        if key == 'ttr' then
+            if val < 0 then
+                error('Invalid value of ttr: ' .. val)
+            end
+        else
+            error('Unknown option ' .. key)
+        end
+    end
+
+    queue.settings = opts
+end
+
 --- Create everything that's needed to work with "shared" sessions.
 local function identification_init()
     local queue_session_ids = box.space._queue_session_ids