feature: unify the variable naming style

[hustjieke]

Is your feature request related to a problem? Please describe.

The names of variables (including function parameters) and data members are all lowercase, with underscores between words. Data members of classes (but not structs) additionally have trailing underscores. For instance: a_local_variable, a_struct_data_member, a_class_data_member_.

Describe the solution you'd like

For example:

std::string table_name;  // OK - lowercase with underscore.
std::string tableName;   // Bad - mixed case.

class TableInfo {
  ...
 private:
  std::string table_name_;  // OK - underscore at end.
  static Pool<TableInfo>* pool_;  // OK.
};

...
...

Describe alternatives you've considered

Additional context

For more details:
https://google.github.io/styleguide/cppguide.html#Variable_Names

[RingsC]

This issue classsified as clean code task, just as #11 does. Therefore, we close this, and reopen if necessary.