Merge #19604: Pass mempool pointer to UnloadBlockIndex/GetCoinsCacheSizeState

fae8c28dae Pass mempool pointer to GetCoinsCacheSizeState (MarcoFalke)
fac674db20 Pass mempool pointer to UnloadBlockIndex (MarcoFalke)
faec851b6e test: Simplify cs_main locks (MarcoFalke)

Pull request description:

  Split out from #19556

  Instead of relying on the implicit mempool global, pass a mempool pointer (which can be `0`). This helps with testing, code clarity and unlocks the features described in #19556.

ACKs for top commit:
  jnewbery:
    code review ACK fae8c28dae
  fjahr:
    Code review ACK fae8c28dae
  darosior:
    Tested ACK fae8c28dae
  jamesob:
    ACK fae8c28dae ([`jamesob/ackr/19604.1.MarcoFalke.pass_mempool_pointer_to`](https://github.com/jamesob/bitcoin/tree/ackr/19604.1.MarcoFalke.pass_mempool_pointer_to))

Tree-SHA512: fa687518c8cda4a095bdbdfe56e01fae2fb16c13d51efbb1312cd6dc007611fc47f53f475602e4a843e3973c9410e6af5a81d6847bd2399f8262ca7205975728

src/qt/test/apptests.cpp

@@ -83,7 +83,7 @@ void AppTests::appTests()
     // Reset global state to avoid interfering with later tests.
     LogInstance().DisconnectTestLogger();
     AbortShutdown();
-    UnloadBlockIndex();
+    UnloadBlockIndex(/* mempool */ nullptr);
     WITH_LOCK(::cs_main, g_chainman.Reset());
 }