Validate the "share root tree" when decoding shares

The URI extension block contains the root hash of a merkle tree ("share root tree") where the leaves are themselves the root hashes of merkle trees where the leaves are the ZFEC outputs ("blocks") belonging to each share. The merkle path from this root hash to the root of the share's own "block hash tree" must be validated. This is possible using the "needed hashes" list included in the share.

Sub-task of #16