client: add support for metalink source
Motivation:
FTS supports bulk operations, but using its own specific format. The
metalink is a well-documented (see RFC 5854) XML format that describes
how to receive a set of files.
Support for metalink would allow FTS to accept a bulk transfer request
with the FTS user presenting the information in a standard fashion.
Modification:
Add initial support for parsing the metalink XML format. The idea is
that the user presents a base URL that all files (within the metalink
file) are resolved against.
There are several limitations with the current implementation:
* Metalink allows for a file to have multiple sources. This patch
selects only the best url (the one with the lowest priority value).
* There are several types of source URLs supported by metalink (http,
bittorrent, etc.). The code currently assumes that any source URL
is acceptable by FTS
* In principle, the metalink allows a file to have different digest
values, as calculated by different checksum algorithms. Currently,
only adler32 checksums are supported.
Result:
It is now possible to submit a bulk request to FTS using a metalink
file.