Merge #21377: Speedy trial support for versionbits

ffe33dfbd4 chainparams: drop versionbits threshold to 90% for mainnnet and signet (Anthony Towns)
f054f6bcd2 versionbits: simplify state transitions (Anthony Towns)
55ac5f568a versionbits: Add explicit NEVER_ACTIVE deployments (Anthony Towns)
dd07e6da48 fuzz: test versionbits delayed activation (Anthony Towns)
dd85d5411c tests: test versionbits delayed activation (Anthony Towns)
73d4a70639 versionbits: Add support for delayed activation (Anthony Towns)
9e6b65f6fa tests: clean up versionbits test (Anthony Towns)
5932744450 tests: test ComputeBlockVersion for all deployments (Anthony Towns)
63879f0a47 tests: pull ComputeBlockVersion test into its own function (Anthony Towns)

Pull request description:

  BIP9-based implementation of "speedy trial" activation specification, see https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-March/018583.html

  Edge cases are tested by fuzzing added in #21380.

ACKs for top commit:
  instagibbs:
    tACK ffe33dfbd4
  jnewbery:
    utACK ffe33dfbd4
  MarcoFalke:
    review ACK ffe33dfbd4 💈
  achow101:
    re-ACK ffe33dfbd4
  gmaxwell:
    ACK ffe33dfbd4
  benthecarman:
    ACK ffe33dfbd4
  Sjors:
    ACK ffe33dfbd4
  jonatack:
    Initial approach ACK ffe33dfbd4 after a first pass of review, building and testing each commit, mostly looking at the changes and diffs. Will do a more high-level review iteration. A few minor comments follow to pick/choose/ignore.
  ariard:
    Code Review ACK ffe33df

Tree-SHA512: f79a7146b2450057ee92155cbbbcec12cd64334236d9239c6bd7d31b32eec145a9781c320f178da7b44ababdb8808b84d9d22a40e0851e229ba6d224e3be747c

test/functional/rpc_blockchain.py

@@ -149,6 +149,7 @@ class BlockchainTest(BitcoinTestFramework):
                         'count': 57,
                         'possible': True,
                     },
+                    'min_activation_height': 0,
                 },
                 'active': False
             },
@@ -158,7 +159,8 @@ class BlockchainTest(BitcoinTestFramework):
                     'status': 'active',
                     'start_time': -1,
                     'timeout': 9223372036854775807,
-                    'since': 0
+                    'since': 0,
+                    'min_activation_height': 0,
                 },
                 'height': 0,
                 'active': True