BIP341: speedy trial activation parameters

Adds mainnet and testnet3 activation parameters. This uses the "speedy trial" approach [0] with some refinements to the exact mechanism [1], as implemented in bitcoin/bitcoin#21377 . Parameters are selected based on previous discussions [2] [3] [4]. The parameters are:

• starttime = 2021-04-24 00:00 UTC
• timeout = 2021-08-11 00:00 UTC
• min_activation_height = 709632 (est mid November 2021, mainnet only)
• threshold = 1815 / 2016 blocks (90%, mainnet only)

In terms of block heights, that will likely mean the first signalling period for mainnet should start at height 681408, approximately April 29th UTC, and will likely mean signalling will continue for 8 retarget periods until height 697536 (which if blocks arrived at 10 minute intervals, would be due around 2021-08-20).

[0] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-March/018583.html [1] https://github.com/bitcoin/bitcoin/pull/21377#issuecomment-814494847 [2] (90%) https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-February/018425.html [3] (May/August/Nov) https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-March/018715.html [4] (specific parameters) https://github.com/bitcoin/bips/pull/1081