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 Job ID Name Coverage
  Preparation
passed #65857
gst indent

00:00:15

passed #65856
manifest

00:00:18

 
  Build
passed #65858
build fedora x86_64

00:01:46

 
  Test
passed #65859
check fedora

00:16:18

 
  Full Builds
passed #65860
build android arm64 api28

00:02:47

passed #65861
build cerbero fedora x86_64

00:10:21