Refactor Orc JIT to enable lazy compilation (#850)

[wenyongh]

Refactor LLVM Orc JIT to actually enable the lazy compilation:
https://llvm.org/docs/ORCv2.html#laziness
Main modifications:

• Create LLVM module for each wasm function, wrap it with thread safe module
  so that the modules can be compiled parallelly
• Lookup function from aot module instance's func_ptrs but not directly call the
  function to decouple the module relationship
• Compile the function when it is first called and hasn't been compiled
• Create threads to pre-compile the WASM functions parallelly when loading
• Set Lazy JIT as default, update document and build/test scripts