job-exec: use flux_job_kvs_namespace() to create namespace name

Switch to using libjob's flux_job_kvs_namespace() to create job
guest kvs namespace names, instead of using the dothex notiation
of a flux_jobid_t.

This puts job namespace name generation in a single place so that
these names do not have to be generated manually anywhere, as
well as allowing the naming convention to be updated in the future
without breaking any existing code.

As a side effect, this fixes a problematic side effect of kvs
namespace names with periods in them, since these names are
used as keys in kvs module stats output.

Fixes #2306

src/modules/job-exec/job-exec.c

@@ -819,13 +819,6 @@ static flux_future_t *jobinfo_start_init (struct jobinfo *job)
     return NULL;
 }
 
-/*  Create namespace name for jobid 'id'
- */
-static int job_get_ns_name (char *buf, int bufsz, flux_jobid_t id)
-{
-    return fluid_encode (buf, bufsz, id, FLUID_STRING_DOTHEX);
-}
-
 static double job_get_kill_timeout (flux_t *h)
 {
     double t = DEFAULT_KILL_TIMEOUT;
@@ -866,7 +859,7 @@ static int job_start (struct job_exec_ctx *ctx, const flux_msg_t *msg)
         jobinfo_decref (job);
         return -1;
     }
-    if (job_get_ns_name (job->ns, sizeof (job->ns), job->id) < 0) {
+    if (flux_job_kvs_namespace (job->ns, sizeof (job->ns), job->id) < 0) {
         jobinfo_fatal_error (job, errno, "failed to create ns name for job");
         flux_log_error (ctx->h, "job_ns_create");
         return -1;

t/t2402-job-exec-dummy.t

@@ -77,7 +77,7 @@ test_expect_success 'job-exec: job exception uses SIGKILL after kill-timeout' '
 	        | flux job submit) &&
 	flux job wait-event -vt 1 $id start &&
 	flux kvs get --waitcreate \
-		--namespace=$(flux job id --to=hex $id) \
+		--namespace=$(flux job namespace $id) \
 		trap-ready &&
 	flux job cancel $id &&
 	sleep 0.2 &&