* core: lookup txs by block number instead of block hash

Transaction hashes now store a reference to their corresponding
block number as opposed to their hash. In benchmarks this was
shown to reduce storage by over 12 GB.

The main limitation of this approach is that transactions on
non-canonical blocks could never be looked up, however that is
currently not supported.

The database version has been upgraded to version 5 and the
transaction lookup process is backwards-compatible with the
prior two transaction lookup formats prexisting in the
database instance. Tests have been added to ensure this.

* core/rawdb: tiny review nit fixes