Lift serialization outside of system DB

[maxdml]

This PR lifts the encoding/decoding of workflows and steps input/outputs outside of the system database, to facilitate further work on serialization. We also make sure to store nil pointers as NULL in Postgres (otherwise an empty string per gob/json + b64 encoding), so we can always return the right value to users (specifically on the ListWorkflows and GetWorkflowSteps path which do not have type information for automatic recasting.)

The system DB functions thus always return pointers to (encoded) strings.

Note that sleep must still call (de)serialize, when reading/writing the wake up time. This won't be a problem because the type is well known. (We could decide to have an independent encoding and/or no encoding for this, but I don't want to introduce heterogeneity in the encoded data living in the DB.)