queue2: Defer downstream bitrate query to the streaming thread.

When we want to perform a downstream bitrate query, just
set the reconfigure flag on the srcpad and get the streaming
thread to do it. That avoids emitting a downstream query
when receiving the upstream RECONFIGURE event - which can
lead to deadlocks if downstream is sending the event from
within a lock - e.g. input-selector.

If querying the downstream bitrate changes the cached
value, then make sure to update our buffering state
and potentially post a BUFFERING message to the application.

Fixes: #566
Part-of: <!501>
29 jobs for master in 28 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
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  Build
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gst-ios-13.2
build cerbero cross-ios universal
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gst-macos-10.15
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1809 windows docker allowed to fail manual
build msys2
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  Test
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  Integrate
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cross-android universal examples
created #2972414
gst-ios-13.2
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created #2972413
documentation