[Network] node falls behind (and catches up) when there is a tenure microfork

[jcnelson]

When the node encounters a microfork, it has a hard time fetching the canonical tenure and keeping up with the canonical tip.

Specifically, if miners in tenure T mine a block at height X, and then tenure T + 1 begins with a tenure-start block that is also at height X, then the node gets stuck until tenure T is confirmed by tenure T + 2.