disregard compiled.prefetch for executemany; optimize default usage; consider dumping most of DefaultRunner

for an executemany we don't set up prefetch_cols or postfetch_cols. we shouldn't look at compiled.prefetch either - we are looping through each parameter set so if there are any python-level defaults, we really have to go through the whole list and look in those keys for None.
if the column default is a constant value and not a callable, just determine that up front before looping through all the parameter sets.
none of this will work well with DefaultRunner, which at this point is a silly object from an ancient line of thinking (that of using "visitor" pattern). dealing with ColumnDefault is entirely an isinstance() game these days. Just move the "visit_sequence()" part of it onto a dialect.execute_sequence() method. The rest of it just shove into Connection or some simpler object.
ColumnDefaults and all that are static objects. We can kill all the calls to isinstance() in compiler and the ones we're adding here by just putting some quickie memoized flags on the object - is_sequence, is_python_callable, is_server_side, etc. that will almost certainly give us some methods back in zoomark.

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