Removed peer authenticated assert in Floodgate::broadcast

[SirTyson]

Description

Resolves https://github.com/stellar/stellar-core-internal/issues/335

This PR removes an expensive assert in Floodgate::broadcast. This assert had to acquire mStateMutex from the main thread, but would often be blocked by the background thread. During periods of high load (1850 TPS), waiting for this mutex would double the median time of broadcast. Because a peer's authenticated state can only be changed from the mainthread anyway, the assert was replaced with a lockless main thread assert. During testing at 1850 TPS, this change resulted in a 15% decrease of txset fetch time, as well as a 10% decrease in nomination latency.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[marta-lokhova]

LGTM! We can merge this in the public repo