SecretStore: remove session on master node (#5545)

* ECDKG protocol prototype

* added test for enc/dec math

* get rid of decryption_session

* added licenses

* fix after merge

* get rid of unused serde dependency

* doc

* decryption session [without commutative enc]

* failed_dec_session

* fixed tests

* added commen

* added more decryption session tests

* helper to localize an issue

* more computations to localize error

* decryption_session::SessionParams

* added tests for EC math to localize problem

* secretstore network transport

* encryption_session_works_over_network

* network errors processing

* connecting to KeyServer

* licenses

* get rid of debug println-s

* fixed secretstore args

* encryption results are stored in KS database

* decryption protocol works over network

* enc/dec Session traits

* fixing warnings

* fix after merge

* on-chain ACL checker proto

* fixed compilation

* fixed compilation

* finally fixed <odd>-of-N-scheme

* temporary commented test

* 1-of-N works in math

* scheme 1-of-N works

* updated AclStorage with real contract ABI

* remove unnecessary unsafety

* fixed grumbles

* wakeup on access denied

* encrypt secretstore messages

* 'shadow' decryption

* fix grumbles

* lost files

* secretstore cli-options

* decryption seccion when ACL check failed on master

* disallow regenerating key for existing document

* removed obsolete TODO

* fix after merge

* switched to tokio_io

* fix after merge

* fix after merge

* fix after merge

* fix after merge

* fix after merge

* fixed test

* fix after merge

* encryption session errors are now fatal

* session timeouts

* autorestart decryption session

* remove sessions on completion

* exclude disconnected nodes from decryption session

* test for enc/dec session over network with 1 node

* remove debug printlns

* fixed 1-of-1 scheme

* drop for KeyServerHttpListener

* Use standard encryption and decryption (as in RPC)

* added some tests

* moved DEFAULT_MAC to ethcrypto

* rpc_secretstore_encrypt_and_decrypt

* serialization with "0x" prefix (RPC compatibility)

* secretstore RPC API

* fix after merge

* fixed typo

* secretstore_shadowDecrypt RPC

* enable secretstore RPCs by default

* fixed test

* SecStore RPCs available without SecStore feature

* fixed grumbles

* lost files

* added password argument to Parity RPCs

* update docs

* remove enc/dec session on master node

* lost file

* pass weak instead of arc