This refactor simplifies and streamlines the code associated with server
side filters. Apart from immediate benefits (clearer code, less duplication)
it is also hoped that this will make it easier to add write filters.
The data pointer dbfl_ref.field can now either point to a copy owned by a
filter, or it can point to the original data owned by a record. In the
latter case, the dbfl_ref.dtor is NULL.
The dbExtractArray* functions are unified to the single function
dbExtractArray and stripped of conversion functionality. This is redundant
because we always call dbGet after applying filters, which takes care of
conversion. Accordingly, dbChannelMakeArrayCopy is now obsolete and its
single use (in the ts filter) replaced with dbExtractArray. Instead, we add
the helper function dbChannelGetArrayInfo to wrap the common boilerplate
around calls to the get_array_info method, used in both arr.c and ts.c.
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