Retrieving metadata from a frontend fail to be saved

[JYA]

When the frontend has finished retrieving the various metadata images it tries to save them on the backend.

If the frontend has a slower connection such as being a wireless client, writing to the backend will fail.

The backend writes a file using a FileTransfer::WriteBlock? call; the code expects all data to be received within 25ms.

Which is very unlikely to occur when saving a typical 300kB image over wireless

[Jean-Yves Avenard <jyavenard@…>]

In 90f47553371e7778ead7354d33baed95b5b56188/mythtv:

Do not abort early if not all data is received within 25ms

0b106b10692ed7efd5eae81b498f591ad39b6222 introduced a 25ms hard limit on how long a transfer could last. This is often too low, especially on wireless frontends

Now read by 50ms blocks and handle smaller reads than requested.
Read will attempt if more than 150ms was spent without receiving any data.

Fixes #11769