statistics: use new api call for detailed stats

statistics: use new api call for detailed stats

the new /statistics/detail api calls takes the type (contact, sender,
receiver) and address for which to display the statistics as explicit
parameter instead of path-component.

This makes it possible to accept '/' as part of an e-mail address
which is allowed (in the local-part by RFC5322 [0], and accepted by
postfix.

[0] https://tools.ietf.org/html/rfc5322

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>