Unify the use of std::log and std::log2

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This is only for clean-up and no functional change.

In the codebase, there are various places that use log() / log(2) and some places mix the use of log and log2. One example is

// logq = log(...)
double loge = logq / log(2) - 2 - log2(p);

This makes the code a bit harder to understand. One easy way is to use std::log2 in all places as it seems more natural in this area.

There are some places that use different log base, such as log(Q.ConvertToDouble()) / log(static_cast<double>(baseKS))). This is not affected by whether we are using log2 or log.