move to "StellarKey" instead of base58

[MonsieurNicolas]

Move to a new format for keys - resolves #599
This also resolves #273 (faster & simpler encoding all around)

Also resolves #623 : the entries in the database and http endpoint are consistently encoded.
XDR objects: stored in base64
Hashes: Hex
AccountIDs: strKey.

note: this is a breaking change for both Horizon (sql schema updated) and for the config files.

I added a command line to help update the configurations:

stellar-core --convertid sfmYZRUn4ofZHzitXupxrrz5QJ7b9mij3rSEdrTNRFVvVgigrcS
Seed:
  strKey: SBI3CZU7XZEWVXU7OZLW5MMUQAP334JFOPXSLTPOH43IRTEQ2QYXU5RG
  base58: sfmYZRUn4ofZHzitXupxrrz5QJ7b9mij3rSEdrTNRFVvVgigrcS
PublicKey:
  strKey: GBA2JPGFUUQ6N4L23K6MUEROQSYMG5BQB3GSFP3KRAXSVGBW7JSV76YL
  base58: gVumxvHdno7nKwNZGK8Bf5bvHrfSNdn8XQ4gdKTXbUXcVax6xE
  hex: 41a4bcc5a521e6f17adabcca122e84b0c374300ecd22bf6a882f2a9836fa655f

also

• fix typo in .x file, resolves enum value KEY_TYPES_ED25519 should be KEY_TYPE_ED25519 #604
• "info" on offline instances, resolves add a command line to get some basic information on the local instance if it's offline #618

[MonsieurNicolas]

As I am staring at the additional changes to move the other database entries that are in hex to base64, it seems weird to me that we would use base64 for the various hashes that we use in the system (hashes are stored also in history in hex form - so we would have to convert those as well).
Maybe I am just used to it, but I kinda like being able to by just staring at a hash in hex match it to a in memory object, also the abbreviated version looks awfully like what git does.