Refactor search and read methods for better performance

[florentx]

• Simplify signature ofModel.search: arguments order, limit and offset become keyword-only, and undocumented argument reverse is abandoned.

• New: Model.search returns a RecordList which is lazily evaluated. API method is called only when needed: if attributes are read or methods are called. It will use search_read API method when it's adequate.

• New: extracting a part of a lazy RecordList will not call API method, for simple use cases like env['account.move'].search([])[10:90]. It will set offset and limit on the prepared search instead.

• Remove undocumentedEnv._web.

• Refactor code for read field formatter.

• Refactor Record and RecordList constructors: get rid of __new__ and simplify.

• Add private method Record._invalidate_cache to implement Record.refresh.