StartWallet was called before service initialization.
After the recent changes this call was moved after initialization, but
the geth system automatically start services.
This meant that `IsStarted()` returned true, although the reactor was
not started, and only after calling `StopWallet()` and `StartWallet()`
again the system would reach the right state.
This commit changes the behavior so that we only check whether the
reactor has been started when calling `IsStarted()` and we allow
multiple calls to `Start()` on the signal service, which won't return an
error (it's a noop if callled multiple times).
Andrea Maria Piana
Roman Volosovskyi
Pedro Pombeiro
Dmitry Shulyak