ReadViewBase::m_ids had better use an initial fixed part

The ReadViewBase::m_ids is a std::vector of 64-bit transaction IDs. It would seem to be better to use something like small_vector<trx_id_t, 16>.

Even better for the case that ReadViewBase::m_low_limit_id and ReadViewBase::m_up_limit_id are close to each other, we could allocate a fixed-size bitmap, say, 512 bits to correspond to values between m_low_limit_id and m_low_limit_id+511.

Perhaps we could define a union of a bitmap and a dynamically allocated std::vector. One bit could distinguish which one is in use, or it could be distinguished by m_up_limit_id-m_low_limit_id>512.

This idea is inspired by https://github.com/enhancedformysql