Fix checksum verification for mismatch algo types

[patrickbrophy]

Closes #3146

When using an origin backed by the XRootD multiuser plugin, the client would crash with a strconv.ParseInt out-of-rangeerror when there was an algorithm mismatch. Additionally, even when the plugin would fallback to a supported algorithm, the client had no way to verify it because the client only computed explicitly requested checksums.

Changes:

• Compute hashes for all known algorithms during transfer so the client can verify against whatever the server actually returns.
• Add length validation for crc32/crc32c parsing to detect and skip mislabled checksums
• Extract duplicated verfication logic from downloadObject/uploadObject into a shared helper function.
• Request all known checksum types from the server via Want-Digest header to maximize chance of getting supported algorithm
• Add unit test for algorithm fallback and mislabeled checksum scenarios.
• Add integration test against real multiuser origin