This allows for a faster config load due to less locking required.
The change is slightly backwards-incompatible. Before, you could manipulate the
globals namespace at a later stage, but disallowing this feels reasonable for
the performance benefit alone (which especially shows on many-core machines).
Apart from that, it's doubtful if doing so is even useful at all as the DSL
provides no mechanism for you to synchronize your operations that may run in
parallel. The data structures itself are protected from race conditions, but
anything implemented on top of this may still be subject to race conditions.
And even if some user has a good reason for doing this, there's a feasible
workaround by creating your own namespace like globals.mutable and using that
instead.
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