With session tracking on the tx_isolation of importance to
connector frameworks, its important that tracking of tx_isolation
does get informed if a user `set session transaction_isolation=X`
as the alias for tx_isolation.

Rather than just implement this for one variable alias, it
is implemented for all aliases.

To assist with this the key hash of session_tracker is now made up of
the offset, not the sys_var pointer.

The impacts of aliases are:
- If track one variable, its alias changes, you get a tracking
  change on the variable you are monitoring.
- If you track two aliased variables of each other, changing
  a variable will have a tracking events for only one of them.

Aliases of Sys_var_bit are not implemented, because we don't
have them, so would've been no way to verify that the new code works.

Post-Review edits by Sergei Golubchik <serg@mariadb.com>