Merge bitcoin/bitcoin#32862: rpc: use CScheduler for relocking wallet and remove RPCTimer

fcfd3db563 remove RPCTimerInterface and RPCRunLater (Matthew Zipkin)
8a1765795f use WalletContext scheduler for walletpassphrase callback (Matthew Zipkin)

Pull request description:

  This removes the dependency on libevent for events scheduled by RPC commands, like re-locking a wallet some time after decryption with walletpassphrase. Since walletpassphrase is currently the only RPC that does this, `RPCRunLater`, `RPCTimerInterface` and all related methods are left unused, and deleted in the second commit. Any future RPC that needs to execute a callback in the future can follow the pattern in this PR and just use a scheduler from node or wallet context.

  This is an alternative approach to #32796, described in https://github.com/bitcoin/bitcoin/pull/32796#issuecomment-3014309449

ACKs for top commit:
  fjahr:
    Code Review ACK fcfd3db563
  achow101:
    ACK fcfd3db563
  furszy:
    ACK fcfd3db563

Tree-SHA512: 04f5e9c3f73f598c3d41d6e35bb59c64c7b93b03ad9fce3c40901733147ce7764f41f475fef1527d44af18f722759996a31ca83b48cb52153795d5022fecfd14

test/functional/wallet_keypool.py

@@ -4,7 +4,6 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test the wallet keypool and interaction with wallet encryption/locking."""
 
-import time
 from decimal import Decimal
 
 from test_framework.test_framework import BitcoinTestFramework
@@ -127,8 +126,10 @@ class KeyPoolTest(BitcoinTestFramework):
         nodes[0].keypoolrefill(3)
 
         # test walletpassphrase timeout
-        time.sleep(1.1)
-        assert_equal(nodes[0].getwalletinfo()["unlocked_until"], 0)
+        # CScheduler relies on condition_variable::wait_until() which does not
+        # guarantee accurate timing. We'll wait up to 5 seconds to execute a 1
+        # second scheduled event.
+        nodes[0].wait_until(lambda: nodes[0].getwalletinfo()["unlocked_until"] == 0, timeout=5)
 
         # drain the keypool
         for _ in range(3):