Introduce sth caching to Letters.pm

Introduce sth caching to Letters.pm

The re-creation of statement handles for the same queries, repeatedly,
seriously degrades the performance of notices.  This introduces new
class-level caching to avoid that problem.

Note: initial provisions
to do the same with caching of COLUMNS information about tables is
also included, but this would probably be most usefully implemented
in C4::Context (or a separate more central module).  The benefits of
caching that info would probably be even greater.  Note that this
proposed implementation might be obsoleted by ORM integration, where
DB structure info would already be cached.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>