main/file.c: Limit media cache usage to remote files.

When testing for the existance of a file, the media cache is searched even if
the file has no chance of being in it.  This can cause performance issues
as the media cache size increases.

As a result, calls to applications like Read and Playback using local files
must scan through the media cache before playing.  Under load and with a
large cache, this can delay the playback of those files.

This patch updates the function that checks for the existance of a file to
only consult the media cache database if the requested file is a remote path.
It introduces a new is_remote_path() function in main/file.c.

ASTERISK-28625  #close
Reported-by:  <kevin@phoneburner.com>

Change-Id: If91137493732d9034dafa381c081c69274a7dcc9