[CSL-13] SetSecret is used only when epoch starts

[jagajaga]

@Gromak

There are at least two problems:

1. If node becomes offline during one of the first k slots, it wants to participate in MPC and it can, but it won't do it in current version;
2. Currently secret stored in MpcStorage is unset when genesis block is processed by this storage. And it's set only at the beginning of epoch. There is a problem, because genesis block for k-th epoch may be processed after the beginning of k-th epoch. For example, if there was a rollback which affected k - 1-th epoch. In this case stored secret will be unset and will never be set again for k-th epoch.

[jagajaga]

@Gromak

Suddenly 2 was fixed by me recently :/