Make access to NameCache atomic. Sometimes needs a lock.

'readBinIface' updates the name cache in a way that is hard to use
with atomicModifyIORef, so this patch introduces a lock for this case.
All other updates use atomicModifyIORef.

Having a single lock is quite pessimistic, so it remains to be seen
whether this will become a problem.  In principle we only need to make
sure that we do not load the same file concurrently (or that it's
idempotent).  In practice we also need to ensure that concurrent reads
do not cancel each other out (since the new NameCache may be based on
an outdated version).

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