Streaming support in DBD::File needs more eyes

Hi all,

triggered by H.Merijn Brand an myself I coded some extensions to
DBI::DBD::SqlEngine and DBD::File.
I'm not 100% happy with the current state of the code, neither with
the tests. Probably some additional eyes of not to busy persons could
help to finalize.

I don't tend to update the documentation until the code design hasn't
finished and passed all tests for DBD::File, DBD::DBM, DBD::CSV with
and without SQL::Statement. Otherwise I might rewrite doc more than
once and I want to save that time.

The current sqlengine trunk of DBI is in production environment for a
currently developed product since week meanwhile without any issues
(okay - DBD::mysql and mysql itself creates issues, but I don't think
they are DBI::DBD::SqlEngine related).

Any help would be great. When I should join IRC in other times than
7:00 UTC and 16:00 UTC, please tell me and I will try to join when I'm
awake.

/Jens