add mutex/lock to all Evoral::SMF methods that use _smf/libsmf, to avoid inadvertent...

add mutex/lock to all Evoral::SMF methods that use _smf/libsmf, to avoid inadvertent corruption via multithreaded access.

Serialization of Session::save_state() will already protect against most of this, but there is really no
good reason why Evoral::SMF's API should require single-threaded/explicit serialization.