just-mr maps: Properly check for missing values in map chain

The root async map in a chain of calls should always be checked
for missing value, which can happen if, e.g., a cycle happens or
a thread gets killed by the system.

Properly handle this by checking explicitly if a value has been
posted. If not, check for cycles where it makes sense (for
example, in the resolving of symlinks), otherwise report any
pending map keys not yet processed.

This is done for all just-mr commands working with async maps.

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