Replace entry_to_bytes with psycopg2's typecast

entry_to_bytes and its friends were called many times (eg.
entry_to_bytes alone was called 40k times while indexing 500
origins with the metadata indexer), and its use of isinstance
used a non-negligible amount of CPU time.

Instead of using *_to_bytes function as post-processing on
all bits of data returned by postgresql, this patch tells psycopg2
to use a new typecast_bytea function when needed (in adapt_conn).
This function deffers the decoding work to psycopg2, which returns a
memoryview, which is turned into bytes.