ssa.c: Reset prdb_epoch on new upstream connection

Currently only conn_dataup.epoch is reset on new
connection establishment.

In case of ACM connecting to ACCESS layer that doesn't
have SMDB, no PRDB will be calculated and thus ACM
prdb_epoch buffer won't be updated and will contain
some old value, that might be different from
conn_dataup.epoch.

This scenario will lead to quering non-existing PRDB
from ACCESS layer.

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>