abstract: reduced increased CPU consumption

Due to the fact that queue_state_fiber called box.ctrl.wait_r*
every millisecond, the idle queue consumed more than 10% of CPU.

The proposed solution makes calls to box.ctrl.wait_r* blocking
until the read/write mode is changed. As a result, in idle mode,
the queue_state_faber is in the suspended state, while the main
fiber (for example, interactive in interactive-mode) is in the
running state.
Thus, CPU consumption is reduced to about ~1%.

More detailed measurements are described in [1].

1. #192

Closes #183

Author: GRISHNOV

queue/abstract/queue_state.lua

@@ -65,7 +65,7 @@ local function create_state_fiber(on_state_change_cb)
         fiber.self():name('queue_state_fiber')
         while true do
             if current == queue_state.states.WAITING then
-                local rc = pcall(box.ctl.wait_rw, 0.001)
+                local rc = pcall(box.ctl.wait_rw)
                 if rc then
                     current = queue_state.states.STARTUP
                     log.info('Queue state changed: STARTUP')
@@ -76,7 +76,7 @@ local function create_state_fiber(on_state_change_cb)
                     log.info('Queue state changed: RUNNING')
                 end
             elseif current == queue_state.states.RUNNING then
-                local rc = pcall(box.ctl.wait_ro, 0.001)
+                local rc = pcall(box.ctl.wait_ro)
                 if rc then
                     current = queue_state.states.ENDING
                     on_state_change_cb(current)