(ITS#8958) 2nd cn=config update blocks slapd while adding subordinate index

[h.b.furuseth@usit.uio.no]

Full_Name: Hallvard Breien Furuseth
Version: 2.4.47, master
OS: Linux x86_64
URL: ftp://ftp.openldap.org/incoming/Hallvard-Furuseth-190119.tgz
Submission from: (NULL) (193.90.50.50)


Use cn=config to add an index to a subordinate database,
then make a 2nd cn=config change.  2nd change waits while
(I assume) indexing, and blocks other slapd operations.
So ldapwhoami during 2nd change takes 5 seconds.

To reproduce: ./bug.sh
To debug, attach gdb in another terminal during 2nd MOD.
gdb -q ../servers/slapd/slapd `cat slapd.pid`

trace.txt shows the 2nd config_back_modify() waiting
in slap_pause_server().