Fixes race condition uncovered by TSAN in BRAPIClient+Assets
Fixes race condition uncovered by TSAN in BRAPIClient+Assets

- replaced objc_sync_enter locks with queue.async(flags: .barrier) blocks

breadwallet/breadwallet-ios,breadwallet/breadwallet-ios,breadwallet/breadwallet-ios,breadwallet/breadwallet-ios,breadwallet/breadwallet-ios