Add start and stop procs and corresponding enum for tracking state transitions

[michaelsbradleyjr]

Related to the concerns in #300.

My proposal:

• In api/common.nim, add an enum:

  type RunState* = enum unstarted, starting, running, stopping, stopped

• Revise proc new*(T: type StatusObject, ...) so that it causes no side-effects, only instantiates StatusObject, with runState field set to unstarted. Since it would not cause any side-effects, it would not need to return the instance wrapped in Result, just the instance itself.
• Introduce start and stop procs:
  • proc start*(self: StatusObject) should cause side-effects, e.g. initializing/opening accounts.sql and finally setting self.runState = RunState.running. It should not involve invocation of self.login() or self.connect(): those are distinct operations that a user of the library (client implementor) is in the best position to decide when they should be called. However, if self.runState != RunState.unstarted, then proc start should raise a Defect return an error result, and it should be documented that proc start should only be called once can be called successfully only once.
  • proc stop*(self: StatusObject) should cause side-effects, e.g. closing accounts.sql and finally setting self.runState = stopped. It should not involve invocation of self.disconnect() or self.logout(): those are distinct operations that a user of the library (client implementor) is in the best position to decide when they should be called. However, if not (self.networkState == NetworkState.disconnected and self.loginState == LoginState.loggedout and self.runState == RunState.running), then proc stop should raise a Defect return an error result, and it should be documented that proc stop should only be called once can be called successfully only once, e.g. in a context leading directly to program exit.

All public API procs should raise a Defect return an error result if self.runState != RunState.running, except for proc start*(self: StatusObject), and of course proc new*(T: type StatusObject, ...).

[michaelsbradleyjr]

Updated the details above after giving them more thought.

[emizzle]

To prevent forcing implementing clients from having to run stop, disconnect, and logout, we should consider a public api convenience proc that does all three, is async, and returns an error result.