This patch introduces a new type called document_signature_t to represent the
signature of a consensus document. Now, each consensus document can have up
to one document signature per voter per digest algorithm. Also, each
detached-signatures document can have up to one signature per <voter,
algorithm, flavor>.
Found by coverity
test_mem_op_hex was leaking memory, which showed up in a few
tests.
Also, the dir_param test had a memleak of its own.
Found by Coverity
