Make lockablescore RW

[TheBlueMatt]

Currently LockableScore only has one lock and then returns a Score that has some mutable methods and some non-mut methods. We really should make LockableScore have both a read and write lock which would make parallel route-finding trivial, rather than somewhat convoluted today.

[TheBlueMatt]

Should be pretty trivial - just duplicate the associated type and lock function on LockableScore, where one is score_lock (or similar) and the other is write_lock (or similar), then only use the read lock when scoring and the write lock when updating.

[jbesraa]

created an initial pr for this #2197