searchidx: respect XAPIAN_FLUSH_THRESHOLD env if set

Xapian documents and respect XAPIAN_FLUSH_THRESHOLD to define
the interval in documents to flush, so don't override it with
our own BATCH_BYTES.  This is helpful for initial indexing for
those on slower storage but enough RAM.

It is unnecessary for -watch and frequent incremental indexing;
and it increases transaction times if -watch is playing "catch-up"
if it was stopped for a while.

The original BATCH_BYTES was tuned for a machine with little
memory as the default XAPIAN_FLUSH_THRESHOLD of 10000 documents
was causing swap storms.  Using document counts also proved an
innaccurate estimator of RAM usage compared to the actual bytes
processed.

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