* accounts: new AddBackends method in manager

* node,cmd/geth: mv accman backend init to cmd/geth

* node,cmd/geth: mv scrypt config downstreawm from node

* accounts: use static buffer size for accman sub chan

minor fix

* accounts,cmd/geth: update accman backends through its event loop

* accounts,node: add comments

* accounts: un-export newBackendEvent

* accounts: use chan instead of wg in newBlockEvent

* node: rename isKeyDirEphem

* accounts,cmd: AddBackends->AddBackend

* accounts: fix potential blocking when adding backend