StatusChange events should be utilized as hints for keepaliver and refiller

[Lorak-mmk]

We can't use those events as exact oracles for node status for multiple reasons:

• Events may be dropped
• Node being up does not always imply that client can connect to it (there may be network issues)
• I think there are other scenarios too

We can still utilize those events as hints.

• When we receive UP event, then we should tell the pool refiller to try to refill immediately instead of waiting (up to 10s) that way we will regain connections quicker
• When we receive DOWN event we should tell the keepaliver to send a keepalive - that way driver will realize quicker that the connection is broken.

[Lorak-mmk]

Btw I think we should only use them as hints, and not store the status in Node at all, because it is not useful.