blockstorage: Add DeletePruneLock

Also adds basic unit test coverage for prune lock management methods.

src/node/blockstorage.cpp

@@ -404,6 +404,12 @@ void BlockManager::UpdatePruneLock(const std::string& name, const PruneLockInfo&
     m_prune_locks[name] = lock_info;
 }
 
+bool BlockManager::DeletePruneLock(const std::string& name)
+{
+    AssertLockHeld(::cs_main);
+    return m_prune_locks.erase(name) > 0;
+}
+
 CBlockIndex* BlockManager::InsertBlockIndex(const uint256& hash)
 {
     AssertLockHeld(cs_main);