added support for different cache methods (currently implemented: build_cache_all() and build_cache_by_seq_region()

methods for initial db checks; two variants of build_cache(): build_cache_by_seq_region() and build_cache_all()

1)  Indicate truncated Xref descriptions by appending ' /.../' to
    the end of them.  The maximum length of the descriptions is 255
    characters (including this truncation marker).

2)  Properly check for successful running of the 'xref_config2sql.pl'
    script and be verbose about reporting failures (from the
    documentation of the 'system' command).

multiple spaces after the words 'source' or 'species' are flagged as
errors.

Reworked recalc_display_xrefs functionality so that it provides a more complete of SQL files (e.g. unset display_xrefs and descriptions, and also gene status updates).

has consequences for the foreign keys-file.

in a feature collection exon as well.