SecretStore: administrative sessions prototypes (#6605)

* generate random channel encryption key on restart

* session-level nonces

* fixed warning after rebase

* session_nonce -> nonce

* full_generation_math_session_with_refreshing_shares && full_generation_math_session_with_adding_new_node

* add serveral secret shares at once

* SecretStore: initial ShareAdd session prototype

* SecretStore: ServersSetChange jobs

* SecretStore: servers set change session continued

* SecretStore: servers set change session continued

* SecretStore: servers set change session continued

* SecretStore: known_sessions iterator

* SecretStore: implemented SessionsQueue

* SecretStore: UnknownSessionsJobTransport

* SecretStore: node_added_using_servers_set_change almost done

* SecretStore: continue adding tests

* SecretStore: node_added_using_servers_set_change + node_added_using_share_add

* SecretStore: node_added_using_server_set_change_from_this_node

* SecretStore: node_moved_using_share_move

* SecretStore: node_moved_using_servers_set_change

* SecretStore: node_removed_using_share_remove

* SecretStore: node_removed_using_servers_set_change

* SecretStore: different folders for client && admin sessions

* SecretStore: started adding share change consensus (flush)

* SecretStore: fixed spurious tests failures

* enum JobPartialRequestAction

* SecretStore: started adding consensus layer to ShareAdd session

* SecretStore: starting external consensus for ShareAdd

* SecretStore: started restoring node_added_using_servers_set_change

* SecretStore: node_added_using_servers_set_change works with external consensus

* SecretStore: node_added_using_server_set_change_from_this_node works with external consensus

* removed debug comments/printlns

* SecretStore: share move session supports consensus

* SecretStore: share remove with external consensus

* SecretStore: started adding basic ShareAdd tests

* SecretStore: added ShareAdd tests

* SecretStore: added ShareAdd session to cluster

* SecretStore: added share move && remove sessions to cluster

* SecretStore: ShareMove session tests cleanup

* SecretStore: ShareRemove session tests cleanup

* SecretStore: added check_secret_is_preserved check

* SecretStore: added servers set change to cluster

* SecretStore: cleaned up ServersSetChange session tests

* SecretStore: cleaning + added tests for ShareRemove

* SecretStore: cleaning up

* SecretStore: propagated admin_public

* SecretStore: fixed persistent_key_storage test

* SecretStore: upgrade_db_from_1

* SecretStore: fixed ServersSetChange session completion

* SecretStore: check polynom1 in ShareAdd sessions (error for pre-v2 shares)

* SecretStore: fixing TODOs

* SecretStore: fixing TODOs

* SecretStore: check share change plan on 'old' slave nodes

* SecretStore: fixing TODOs

* SecretStore: store all admin sessions in single container to avoid overlaps

* SecretStore: do not update nodes set during admin sessions

* SecretStore: moved TODOs to appropriate methods

* SecretStore: TODOs

* SecretStore: added admin_public arg && fixed warnigs

* SecretStore: added shares_to_move_reversed to ShareMove session

* SecretStore: additional checks during consensus establishing

* license

* SecretStore: added TODO about starting ServersSetChange session

* SecretStore: redundant clones + docs + lsot unimplemented-s

* SecretStore: generation_session_completion_signalled_if_failed_on_master

* SecretStore: updated obsolete comment

* SecretStore: added type alias for current DocumentKeyShare serialization format

* SecretStore: fixed typo

* SecretStore; fixed warnings for futures 0.1.15

* fixed warning