fix: serialize NetworkTransport data race on ServerAddr()

[moogacs]

ServerAddr() is called concurrently through AppendEntries, RequestVote.
for example getConn() is thread safe here by calling connPoolLock, however getProviderAddressOrFallback() is not thread safe.

given that the lib doesn't control which data type is used by the consumer and there is no documentation for how ServerAddr() could be used by RAFT lib, so this PR protecting reading ServerAddr() by adding RWMutex to the NetworkTransport and serialize access to serverAddressLock

[jmurret]

Thank you @moogacs !

[gavraz]

Hey,
I came across this PR and noticed and noticed there is no invocation of serverAddressLock.Lock which suggests this has no effect since we only handle reads.