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:

#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
        user	0m0.014s
        sys	0m7.170s

As side effect, this corrects the journal's rate limiter feature: we now
always use the unit name as key for the ratelimiter.

Makefile.am

@@ -4519,6 +4519,8 @@ libjournal_core_la_SOURCES = \
 	src/journal/journald-audit.h \
 	src/journal/journald-rate-limit.c \
 	src/journal/journald-rate-limit.h \
+	src/journal/journald-context.c \
+	src/journal/journald-context.h \
 	src/journal/journal-internal.h
 
 nodist_libjournal_core_la_SOURCES = \

src/basic/string-util.h

@@ -200,3 +200,10 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(char *, string_free_erase);
 #define _cleanup_string_free_erase_ _cleanup_(string_free_erasep)
 
 bool string_is_safe(const char *p) _pure_;
+
+static inline size_t strlen_ptr(const char *s) {
+        if (!s)
+                return 0;
+
+        return strlen(s);
+}

src/journal/journald-audit.c

@@ -413,7 +413,7 @@ static void process_audit_string(Server *s, int type, const char *data, size_t s
                 goto finish;
         }
 
-        server_dispatch_message(s, iov, n_iov, n_iov_allocated, NULL, NULL, NULL, 0, NULL, LOG_NOTICE, 0);
+        server_dispatch_message(s, iov, n_iov, n_iov_allocated, NULL, NULL, LOG_NOTICE, 0);
 
 finish:
         /* free() all entries that map_all_fields() added. All others