crimson/os/seastore: fix laddr_t formatter and its use

'laddr_t' existing formatter did not support a ':x' format specifier
(actually - the output was always hexadecomal).
Here we remove the ':x', but also refactor the custom formatter to
avoid using the streambuf mechanism.
Note - SEASTORE_LADDR_USE_BOOST_U128 is no longer supported by the formatter.

Fixes: https://tracker.ceph.com/issues/77399
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>