Use ShardingSphereIdentifier to handle case-sensitive or case-insensitive for different databases

[terrymanu]

The key of String on CaseInsensitiveMap can not handle identifier with quote and different of database dialects.
It is better to create a new class to replace all CaseInsensitiveMap in metadata module.

The tasks are:

1. Create ShardingSphereIdentifier to instead of all String key of CaseInsensitiveMap/HashMap in metadata module;
2. Create DatabaseDialectIdentifierHandler to process different database dialects.
3. Use ShardingSphereIdentifier on ShardingSphereTable/ShardingSphereSchema/ShardingSphereDatabase etc.
4. Use ShardingSphereIdentifier on rules.
5. Add more implementations for DatabaseDialectIdentifierHandler.