Buffer getter methods return internal references instead of copies

[aristidesstaffieri]

What version are you using?

latest

Problem

File: internal/indexer/indexer_buffer.go (multiple methods)

Description: GetTrustlineChanges(), GetContractChanges(), GetAccountChanges(), GetSACBalanceChanges(), and GetStateChanges() return direct references to internal data structures. Compare with GetTransactionsParticipants() which correctly uses maps.Clone(). While currently safe because data is consumed synchronously before Clear(), this violates the buffer's thread-safety contract and is fragile.