Reduce contention in CatalogManager::ScopedLeaderSharedLock

CatalogManager::ScopedLeaderSharedLock::ScopedLeaderSharedLock() currently holds a spinlock while accessing consensus->ConsensusState(). That call makes a copy of a protobuf, which requires allocation and a later deallocation. Every master lookup RPC needs to go through this path, so this can become a contention point under heavy multi-client load.