Add `timeout` to all store configs and use it in Context

[glimchb]

          Something else came to mind: With some store configs having a timeout parameter, we should either document that they are completely ignored, _or_ in the `WithContext` methods we could check for an existing ctx timeout, and when one is set we leave it as is, but when none is set we derive a new ctx with the timeout from the store config.

Originally posted by @philippgille in #136 (comment)