TokenMap computation takes too long for large clusters

Description

From this message (posted on the cassandra user ML) we can see that the tokenmap build process took 19 seconds which is too slow even though there are 100+ hosts with 256 vnodes each.

This affects the cluster startup time.

Similar to

Also see

At the very least we should remove the duplicate code here https://github.com/datastax/csharp-driver/blob/43037b9c62d430e397aadf38499128b382552656/src/Cassandra/TokenMap.cs#L152-L164
(UpdateKeyspace has the exact same lines of code pretty much)

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Reporter

Joao Reis

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