Add eosio_assert_code intrinsic

get_error_message now returns safer optional<string> even though it may introduce unnecessary copies; find_error_message no longer necessary

Fix regproxy, unregproxy, and cancel delay of cleos system subcommand

Enable claim rewards unit test before 150000000 is staked

Fix ubuntu version check ( OS_MAJ -lt 16 )

Add unlock-timeout option to wallet unlock command

add test case for uncover area of transaction.cpp (2773)

fix-Inssue3194  The old document wallet port is wrong, the correct port is 8888, the number of 8889 in the document

Adjust voting weight

Rename "EOS" symbol in eosio.system to "SYS"

if a user specifies the unlock-timeout option to the wallet unlock command, cleos runs keosd with --unlock-timeout option

Also added support in `abi_serializer` for getting the error message
corresponding to an error code.

- also exempt the eosio account from this restriction