Reduce # number DeviceReadBuffer::Read() calls.

Refs #11252.

This reduces the number of reads per second we make in the stream handlers
utilizing the DRB. It does this by waiting 20 ms for at least dev_read_size
bytes rather than 500 ms for min_read bytes. The device reads are larger
than the minimum read so this actually results in waiting a bit longer
under normal conditions while returning more quickly when we're not getting
data from the device. A win-win.

mythtv/libs/libmythtv/recorders/DeviceReadBuffer.cpp

@@ -625,7 +625,7 @@ bool DeviceReadBuffer::CheckForErrors(
  */
 uint DeviceReadBuffer::Read(unsigned char *buf, const uint count)
 {
-    uint avail = WaitForUsed(min(count, (uint)min_read), 500);
+    uint avail = WaitForUsed(min(count, (uint)dev_read_size), 20);
     size_t cnt = min(count, avail);
 
     if (!cnt)