Cache Mail::Address->parse

calling it repeatedly, for example from ArticleGet can be quite
costly, and ArticleGet calls it two to three times (indirectly)
for each email address, and potentially many times more if the same
email address appears multiple times.

Since Kernel::System::EmailParser objects are short-lived and
Mail::Address objects rather small, the additional memory use
is not a problem.

With warm caches, this reduces the time spent in ArticleGet from
420ms to 230ms on a fairly large ticket.
Rename new method to _MailAddressParse

as suggested by @mgruner
Merge branch 'cache-mail-parsing' of https://github.com/moritz/otrs
Improved mail address parsing speed, thanks to Moritz Lenz.
Fixed POD.