Preserve mailbox on child respawn

[onsails]

Currently on a child failure children will spawn another child. Previous child's mailbox will be disposed. It doesn't fit into let-it-fail concept.

We need childs to preserve mailbox on respawn. Also we need a lifecycle callbacks which can access child state and a message which caused child failure.

[vertexclique]

Like we have talked, let's go with flat hierarchy for this.

[untoldbyte]

if it is of any help. I started looking outside of actix cause it doesnt support this. its very important to have this to truly mimic the fautl tolerance of BEAM

I speak this as an end-user and not a developer

[Relrin]

Done in PR #199.