tools: Add support for submitting multiple entries efficiently

The current sigsum-submit tool only submits one leaf at a time, and waits for a properly cosigned tree head and inclusion proof. To submit multiple leaves (e.g., the hashes of each file in a directory), it would be more efficient to first submit add-leaf requests for all the leaves, then wait for a cosigned tree head including some or all of the submitted leaves, and collect the inclusion proofs.

Add support for this both to the command line tool, and underlying library code.