xlog: allow to extend inprogress xlog file cleanup

We call xdir_collect_inprogress() at startup to clean up the xlog
directory of files left from the previous run. Let's rename it to
xdir_remove_temporary_files() and make it delete all files for which
the new callback function xlog_file_is_temporary() returns true. By
default, the callback returns true only for .inprogress files but it
can be overridden to make xdir_remove_temporary_files() delete other
kinds of files. This is required for thorough file deletion.

Needed for tarantool/tarantool-ee#540

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring