journald: add minimal client metadata caching

      Cache client metadata, in order to be improve runtime behaviour under
pressure.

This is inspired by @vcaputo's work, specifically:

https://github.com/systemd/systemd/pull/2280

That code implements related but different semantics.

For a longer explanation what this change implements please have a look
at the long source comment this patch adds to journald-context.c.

After this commit:

        # time bash -c 'dd bs=$((1024*1024)) count=$((1*1024)) if=/dev/urandom | systemd-cat'
        1024+0 records in
        1024+0 records out
        1073741824 bytes (1.1 GB, 1.0 GiB) copied, 11.2783 s, 95.2 MB/s

        real	0m11.283s
        user	0m0.007s
        sys	0m6.216s

Before this commit:

        # time bash -c 'dd bs=$((1024*1024)) count=$((1*1024)) if=/dev/urandom | systemd-cat'
        1024+0 records in
        1024+0 records out
        1073741824 bytes (1.1 GB, 1.0 GiB) copied, 52.0788 s, 20.6 MB/s

        real	0m52.099s
        us