Add start date parameter to zcoin activation request

[shamardy]

For resource-constrained devices, such as mobiles, it would be beneficial to allow the user to specify the start date from which to begin syncing zcoin blocks. The challenge lies in obtaining the actual block information with its corresponding real block time because we can't fetch the starting block based on an arbitrary time. To address this, we can sync backwards from the latest block using a defined step/range until we find a block with a timestamp earlier than the start date specified in the request.

[shamardy]

Fixed in #1922