Merge pull request #2885 from bas-vk/subscriptions

core/types/block.go

eth/downloader/api.go

eth/filters/api.go

eth/filters/api_test.go

eth/filters/filter.go

eth/filters/filter_system.go

eth/filters/filter_system_test.go

internal/ethapi/api.go

rpc/notification.go

rpc/server.go

rpc/server_test.go

rpc/subscription.go

rpc/subscription_test.go

rpc/types.go

rpc/utils.go