Move statement_number from module to Connection
Move statement_number from module to Connection

Each db session has it's own statement and portal namespace, so we can
keep statement_number and portal_number at connection level rather than
module level. This should reduce lock contention for cases where there's
single module and lots of connections.

kaniini/pg8000,realazthat/pg8000,tlocke/pg8000,kaniini/pg8000,realazthat/pg8000