camera: add a lock for the request tracker

The request tracker manages "in flights" buffers and frames. These
two containers are modified by calling the tracker's member functions.
Such functions are called from at least three threads: (see
`v4l2_camera.cpp` for more details):
 - "main" thread;
 - separate buffer enqueue thread ("Enqueue buffers");
 - separate buffer dequeue thread ("Dequeue buffers").
It is proposed to introduce additional mutex for the request tracker
to prevent a race condition.

An alternative approach is introducing an additional mutex for the
shared containers inside the `RequresTracker` class. But such approach
requires more changes and may lead to problems with design in the
future.

This commit contains a few additional changes which are not directly
related to the topic:
 - as far as more than one local lock is used all of them are renamed;
 - the `mFlushLock` member is removed because it is completely covered
   by the new mutex.

Test: try to use a camera with more than one buffer and get SIGSEGV
      crash of the `cameraserver` in the `RequestTracker` class.
Change-Id: I2c1901722289cee9f0cb05a11a4dabe70d18f2ba
Signed-off-by: Sergii Piatakov <sergii.piatakov@globallogic.com>

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