horizonclient v4.0.0 & txnbuild v4.0.0

Added support for the new operations in Protocol 14. Now it is possible to:

• Create and claim claimable balance operations (see CAP-23) with the {Create,Claim}ClaimableBalance structures and their associated helpers
• Begin/end sponsoring future reserves for other accounts (see CAP-33) with the {Begin,End}SponsoringFutureReserves operations
• Revoke sponsorships of various objects with the RevokeSponsorship operation (see CAP-33).

Added new client methods and effects supporting Protocol 14:

• New client methods
  • ClaimableBalances(req ClaimableBalanceRequest) - returns details about available claimable balances, possibly filtered to a specific sponsor or other parameters.
  • ClaimableBalance(balanceID string) - returns details about a specific, unique claimable balance.
• New effects:
  • ClaimableBalance{Created,Updated,Removed}
  • ClaimabeBalanceSponsorship{Created,Updated,Removed}
  • AccountSponsorship{Created,Updated,Removed}
  • TrustlineSponsorship{Created,Updated,Removed}
  • Data{Created,Updated,Removed}
  • DataSponsorship{Created,Updated,Removed}
  • SignerSponsorship{Created,Updated,Removed}

Also:

• Removed JSON variant of GET /metrics, both in the server and client code. It's using Prometheus format by default now.
• Added NextAccountsPage.
• Fixed Fund function that consistently errored.
• Added support for Go 1.15.

Breaking changes

• Dropped support for Go 1.13.
• Add support for SEP-10 v2.0.0.
  • Replace BuildChallengeTx's anchorName parameter with homeDomain.
  • Add homeDomain parameter to ReadChallengeTx, VerifyChallengeTxThreshold, and VerifyChallengeTxSigners.