allow customize cache settings

[newgene]

Current default cache setting (7 days) is defined in BaseAPIHandler:

https://github.com/biothings/biothings.api/blob/7131379975e8f8aafafa5858ea274dc8181bc863/biothings/web/handlers/base.py#L67

To make it more flexible to customize, we can allow

• set a global caching time in config.py to override the default value.

  API_HANDLER_CACHE = 3600
  

• set caching time on per-handler base, likely in initialize method. Similar to how biothings_type parameter was set:

https://github.com/biothings/biothings.api/blob/7131379975e8f8aafafa5858ea274dc8181bc863/biothings/web/handlers/query.py#L57

it can be set like:

(r"/api/query/?", "biothings.web.handlers.QueryHandler", {"biothing_type": "schema", "cache": 3600}),