Allow incremental rescan of shielded i/o kernels by remote nodes

[dbadol]

Feature request
Allow wallets using remote nodes (without owner key) to request an incremental rescan of shielded i/o kernels only, as described in BeamMW/beam-ui#1124

Objective
The objective is to allow fast rescan of the shielded pools, so that even when using a remote node (or random node in the case of mobile wallet) the users's wallet can identify and display their latest received offline transactions.

Remarks
Incremental means: from a given block height (i.e. the one of the latest full rescan or mini-rescan of this type).