souphttpsrc: check difference in time from the last socket read before changing blocksize

If the pipeline consumes the data slower than the available network speed,
for example because sync=true, is useless to increase the blocksize and
reading in too big blocksizes can cause the connection to time out

Closes #463
6 jobs for souphttpsrc in 31 minutes and 47 seconds (queued for 52 seconds)
Status Name Job ID Coverage
  Preparation
passed gst indent #65857

00:00:15

passed manifest #65856

00:00:18

 
  Build
passed build fedora x86_64 #65858

00:01:46

 
  Test
passed check fedora #65859

00:16:18

 
  Full Builds
passed build android arm64 api28 #65860

00:02:47

passed build cerbero fedora x86_64 #65861

00:10:21