Replace unique_ptr with raw pointer in expression implementation

[Aiee]

1. Replace all unique_ptr(s) with raw pointers in expression implementation and manage object lifetime using ObjectPool.
2. Disable default expression constructors to preventing mixing raw pointer and unique_ptr.

TODOs:

• Replace expression constructor with static functions, specifying expression type, and add expression into ObjectPool during the construction.
• Split expression UTs to separate files
• Modify other related common UTs
• Encode/Decode test

[Shylock-Hg]

The raw pointer is too implicit for the lifetime management. You could force everyone remember all object in expression is managed by objpool, but when these object referenced by other or passed from others it's more confuse.

[Aiee]

The raw pointer is too implicit for the lifetime management. You could force everyone remember all object in expression is managed by objpool, but when these object referenced by other or passed from others it's more confuse.

After off-line discussion, this scenario could be solved by changing all expression-related members from unique_ptrs to raw pointers.

[CLAassistant]

All committers have signed the CLA.

[yixinglu]

ltgm