blockstorage: Add DeletePruneLock

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

src/node/blockstorage.h

@@ -455,6 +455,9 @@ public:
     //! Create or update a prune lock identified by its name
     void UpdatePruneLock(const std::string& name, const PruneLockInfo& lock_info) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+    //! Delete a prune lock identified by its name. Returns true if the lock existed.
+    bool DeletePruneLock(const std::string& name) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     /** Open a block file (blk?????.dat) */
     AutoFile OpenBlockFile(const FlatFilePos& pos, bool fReadOnly) const;