Partial revert of c64141b0c0b7118ee28be2441374727a1af7c44c

We can use the previous readblocksize as it was adjusted during earlier reads. However we set the initial value in such a way that it will be automatically recalculated on the first call to RingBuffer::CalcReadAheadThresh()
Cache the size of the remote file for 500ms

Aim is to speed up common usage…