plugin/logs:  Pass server txn to mask Eval()
plugin/logs:  Pass server txn to mask Eval()

We were only using it on the prepare step, but we needed to use the
passed in txn each time we eval the prepared query.

Signed-off-by: Patrick East <1a015b2c287ee8e96f2ec59ce4953b89b37de4a8@gmail.com>

tsandall/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa