[minor] fix keyring io.Reader copy in api
[minor] merge pgp/sign and pgp/verify packages into main pgp
[major] refactor scheduler PGP keyring management

Similar to the work done on the api, instead of managing investigator's keys
with a gpg keyring on disk, pull the keys from the database and manage a
keyring in memory.
The addition here is the management of the scheduler's own investigator
identity, that's used during the upgrade protocol to kill old agents. That private
key is also stored in the database, with permissions that limit the access to the
column to the schedulers themselves.
[minor] split up api pgp code into separate file