Fixes#1614.
Prior to this commit, if data in the persisted cache in the wallet file
were wrong (should be a very extraordinary case), then the joinmarket
code would have to crash with a cache invalid warning. After this
commit, in such an extraordinary case, the option exists to invalidate
or remove the cache on startup, so that it can be rebuilt from scratch.
This is done with a config var wallet_caching_disabled in the POLICY
section.