ENH: rewrite ma.median to improve poor performance for multiple dimensions

many masked median along a small dimension is extremely slow due to the
usage of apply_along_axis which iterates fully in python. The unmasked
median is about 1000x faster.

Work around this issue by using indexing to select the median element
instead of apply_along_axis.

Further improvements are possible, e.g. using the current np.nanmedian
approach for masked medians along large dimensions so partition is used
instead of sort or to extend partition to allow broadcasting over
multiple elements.

Closes gh-4683.

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