Make sure that default timeout settings are based on configuration
parameters. This now applies for core data sources using old TSDB
contracts and new SDK contracts. Before it was only applied for old TSDB
contracts.
Also moves global setting variables to non-global (setting.Cfg).
Uses new httpclient package from grafana-plugin-sdk-go introduced
via grafana/grafana-plugin-sdk-go#328.
Replaces the GetHTTPClient, GetTransport, GetTLSConfig methods defined
on DataSource model.
Longer-term the goal is to migrate core HTTP backend data sources to use the
SDK contracts and using httpclient.Provider for creating HTTP clients and such.
Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>