When enabling system variables tracker, make a copy of
global_system_variables.session_track_system_variables
under LOCK_global_system_variables. This protects from concurrent variable
updates and potential freed memory access, as well as from variable
reconstruction (which was previously protected by LOCK_plugin).

We can also use this copy as a session variable pointer, so that we don't
have to allocate memory and reconstruct it every time it is referenced.

For this very reason we don't need buffer_length stuff anymore.

As well as don't we need to take LOCK_plugin early in ::enable().
Unified ::parse_var_list() to acquire LOCK_plugin unconditionally.
For no apparent reason it wasn't previously acquired for global
variable update.

Part of MDEV-14984 - regression in connect performance