Add support for ZIP 313

[nighthawk24]

ZIP 313 changes the default/conventional fee for shielded transactions from 10,000 zats to 1,000 zats
zcash/zips#408

To implement this change, clients are recommended to change the default fee from 10,000 to 1,000 zats for shielded transactions.

Activation:

• The new default fee of 0.00001 or 1000 zats must start activation at block 1,080,000
• With a grace period of ~4 weeks (block 1,120,000) to upgrade to the reduced default transaction fee for zcashd and core clients used by exchanges & service providers.

The implementation to activate the new fee can start anytime from the start to the end of the 4 week transition period outside which the shielded transaction fees will stand out from the updated light client fees of 1000 zats.