Restructure the source code layout

[huachaohuang]

The architecture of Engula can be regarded as three sub-systems: engula, stream-engine, and object-engine. While engula is dedicated to providing Engula specific functionalities, stream-engine and object-engine are generally independent and can be potentially used outside of the Engula project. So to make the project structure clearer, I propose the following project structure:

root/
  engula/
    engula/
    common/
    supervisor/
    transactor/
    cooperator/
    ...
  stream-engine/
    common/
    master/
    ...
  object-engine/
    common/
    master/
    ...

instead of the current layout:

root/
  src/
    engula/
    common/
    supervisor/
    transactor/
    cooperator/
    object-engine/
      common/
      master/
      ...
    stream-engine/
      common/
      master/
      ...

[huachaohuang]

Maybe this is easier without moving stream-engine and object-engine:

root/
  src/
    engula/
      ..
    stream-engine/
      ...
    object-engine/  
      ...

[huachaohuang]

I think there is still some confusion in the current source code layout. Specifically, the relationship between engula-apis, engula-client, and the other crates in the engula directory is not obvious. And using the name engula as a subsystem name is ambiguous with the project name. So I suggest a new layout as follow:

src/
  apis/
  client/
  kernel/
    common/
    supervisor/
    transactor/
    ...
  object-engine/
    ...
  stream-engine/
    ...
``

The `engula-apis` crate is actually an external crate belonging to the `engula/engula-apis` project. And the `engula-client` crate may be moved to a standalone project sooner or later, like other client projects.