all: Update codebase to conform to new XDR types

[Shaptic]

Breakdown and Rollout: Horizon & Protocol 19. See #4261 for the master issue.

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CAP-21 introduces a change to how time is represented in the protocol and it also changes the way you access transaction preconditions. The relevant changes are described in the XDR diff. When a project starts using the updated XDR, it must first be updated to conform to the new predicate time types as well as the new way transaction conditions are accessed. In summary,

• relBefore is a Duration rather than an Int64
• absBefore is a TimePoint rather than an Int64
• timebounds preconditions are accessed via tx.Cond.TimeBounds rather than tx.TimeBounds
• similarly, transactions w/ timebounds should be constructed on the Cond field with the PreconditionTypePrecondTime type

• Go: all: Add Protocol 19 XDR and update StrKey to support Signed Payloads #4279
• Java: Protocol 19 - CAP-40 Signed Payload Signer lightsail-network/java-stellar-sdk#414
• JavaScript: Adding builder methods for new preconditions. js-stellar-base#513