client could support accepting fully qualified upload directory as input to MPU operations

[jordanhendricks]

With the integration of MANTA-3427, the prefix length on MPU directories will be a tunable. As it today, the client assumes a prefix length of 1. Issue #325 will allow the client to resolve the prefix length, allowing tools that just use the upload ID to continue to work.

Because long-lived client sessions likely don't want to incur an additional latency hit to resolve the fully qualified upload directly path for each MPU operation, this change will allow additional flexibility: providing the fully qualified path to the client directly.

This change will be a minor version bump, as it will augment the API in a backwards compatible manner.

[jordanhendricks]

CR is at: https://cr.joyent.us/#/c/2592/.

Testing Notes

• This change is make check clean.
• Ran the test suite using make test.
• Ran the muskie test suite with a version of node-manta using this change, and it passes.