[source debug] refine some code in source debugging (#856)

- move the wait_cond from exec_env to debug_instance, so the debug thread can be waken up by any threads
- process more general query message from debugger
- refine debug instance create/destroy mechanism
- avoid creating debug instance during module instantiating
- avoid blocking execution thread during creating debug instance
- update related documents

doc/build_wamr.md

@@ -146,6 +146,10 @@ Currently we only profile the memory consumption of module, module_instance and
 > Note: The WAMR application entry (`core/iwasm/common/wasm_application.c`) encapsulate some common process to instantiate, execute the wasm functions and print the results. Some platform related APIs are used in these functions, so you can enable this flag to exclude this file if your platform doesn't support those APIs.
 > *Don't enable this flag if you are building `product-mini`*
 
+#### **Enable source debugging features**
+- **WAMR_BUILD_DEBUG_INTERP**=1/0, default to 0 if not set
+> Note: There are some other setup required by source debugging, please refer to [source_debugging.md](./source_debugging.md) for more details.
+
 **Combination of configurations:**
 
 We can combine the configurations. For example, if we want to disable interpreter, enable AOT and WASI, we can run command: