centralize tx size computation across tx subsytem and overlay

[marta-lokhova]

Multiple subsystems in core rely on transaction size computation. For these systems to interact correctly, they must compute tx size in the same way. We've already encountered bugs related to the fact that core has several places where the same computation is done, e.g. in overlay:

stellar-core/src/overlay/FlowControlCapacity.cpp

Line 223 in 0448bf0

return static_cast<uint64_t>(xdr::xdr_size(msg) -

and in tx the subsystem:

stellar-core/src/transactions/TransactionFrame.cpp

Line 227 in 0448bf0

int64_t txSize = xdr::xdr_size(mEnvelope);

We should centralize this logic in one util function to prevent footguns.