Replace SaveJSON and LoadJSON in siadir

MERGE REQUEST

Changes to Work Flow

This addresses the follow up item from !3357 (merged) that added the use of the writeaheadlog to the siadir. !3357 (merged) focused on the wal updates and applying those updates but still relied on SaveJSON and LoadJSON to interact with the disk which was unnecessary since both the wal and SaveJSON/LoadJSON were ACID.

The siadir now just uses the production dependencies to interact with the disk.

Closes #3361 (closed)

Testing Updates

N/A

Modules and Packages Impacted

• modules/renter/siadir