[#1165] Steps 1 & 2 - Download note commitment tree data from lightwalletd

[LukasKorba]

Closes #1165
Closes #1166

• UpdateSubtreeRootsAction added, ensuring the roots are downloaded and stored in the DB
• added ZcashError for putSaplingSubtreeRoots failure
• demo app config temporarily updated to Nighthawk server

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[HonzaR]

The code changes look good to me.

Not sure if this is also the iOS side related, but on the Android, we also need to recognize the error which came from the getSubtreeRoot to retry in case of connectivity issue and not just state that the node does not support the non-linear sync mechanism later, as this action is not part of the regular sync loop.

[LukasKorba]

The code changes look good to me.

Not sure if this is also the iOS side related, but on the Android, we also need to recognize the error which came from the getSubtreeRoot to retry in case of connectivity issue and not just state that the node does not support the non-linear sync mechanism later, as this action is not part of the regular sync loop.

Good catch, I filed the issue #1179