performance modify rate: reduce lock contention on the object extension factory

[tbordaz]

Issue Description
/opt/ldapbench/bin/modrate --hostname instance --port 389 --bindDN 'cn=Directory Manager' --bindPassword xxx --entryDN id=user.[1-10000],ou=People,dc=example,dc=com" --attribute street --attribute description --valueLength 12 --numThreads 5

During these modifies, target entry is allocated/duplicated/free. entry structure contains a object (entry) extension that is allocated via a factory slapi_register_object_extension.
ATM the purpose of this factory is to store "unhashed#user#password" in case of update of the password (pw_exp_init).

The factory is protected via a NSPR simple mutex and is the second hottest lock in contention

#        ----------- Cacheline ----------    Total      Tot  ----- LLC Load Hitm -----  ---- Store Reference ----  --- Load Dram ----      LLC    Total
# Index             Address  Node  PA cnt  records     Hitm    Total      Lcl      Rmt    Total    L1Hit   L1Miss       Lcl       Rmt  Ld Miss    Loads
# .....  ..................  ....  ......  .......  .......  .......  .......  .......  .......  .......  .......  ........  ........  .......  .......
#
      0      0x7f84e9c28500     0     158     1877   16.32%       31       31        0      838      838        0         0         0        0     1039
      1      0x7f84e9c2a980     0      71      109    5.79%       11       11        0       29       29        0         0         0        0       80
      2      0x7f84e9c28340     0      49       77    4.21%        8        8        0       14       14        0         0         0        0       63
      3      0x7f84d74e1d00     0     223      308    3.68%        7        7        0      210      210        0         0         0        0       98
      4      0x7f84e4c18600     0      70       99    3.16%        6        6        0       34       34        0         0         0        0       65

#        ----- HITM -----  -- Store Refs --  --------- Data address ---------                      ---------- cycles ----------    Total
#   Num      Rmt      Lcl   L1 Hit  L1 Miss              Offset  Node  PA cnt        Code address  rmt hitm  lcl hitm      load  records

  -------------------------------------------------------------
      1        0       11       29        0      0x7f84e9c2a980
  -------------------------------------------------------------
           0.00%    0.00%   55.17%    0.00%                 0x0     0       1      0x7f84ee04ea86         0         0       189       28
            |
            ---__GI___pthread_mutex_lock (inlined)
               PR_Lock
               |
               |--30.92%--factory_destroy_extension
...
                --23.85%--factory_create_extension

Moving this mutex to a pthread mutex reduce the contention to be 5th hottest

#        ----------- Cacheline ----------    Total      Tot  ----- LLC Load Hitm -----  ---- Store Reference ----  --- Load Dram ----      LLC    Total
# Index             Address  Node  PA cnt  records     Hitm    Total      Lcl      Rmt    Total    L1Hit   L1Miss       Lcl       Rmt  Ld Miss    Loads
# .....  ..................  ....  ......  .......  .......  .......  .......  .......  .......  .......  .......  ........  ........  .......  .......
#
      0      0x7f0850628600     0     194     2192   18.38%       34       34        0      967      967        0         0         0        0     1225
      1      0x7f0850628440     0      53       83    7.03%       13       13        0       29       29        0         0         0        0       54
      2      0x7f084b618700     0      74      105    5.41%       10       10        0       23       23        0         0         0        0       82
      3      0x7f085062c500     0      21       27    5.41%       10       10        0        0        0        0         0         0        0       27
      4      0x7f0850628380     0      48       92    3.78%        7        7        0       29       29        0         0         0        0       63

  -------------------------------------------------------------
      4        0        7       29        0      0x7f0850628380
  -------------------------------------------------------------
           0.00%    0.00%   31.03%    0.00%                 0x0     0       1      0x7f0854a41963         0         0       112       28         6  [.] __pthread_mutex_lock        libpthread-2.28.so  __pthread_mutex_lock+67          0
            |
            ---__GI___pthread_mutex_lock (inlined)
               |
               |--22.31%--factory_destroy_extension
...
                --8.28%--factory_create_extension

This change gives a 1% increase of the throughput: 3400 vs 3370/s
This change will also impact all others object extension (conn, op, entry,...)

Note: I think another ticket should be created to avoid creating systematically an entry extension. If possible it could rather been created on demand (if unhashed password is managed, if MODs update userpassword)
Package Version and Platform:
all version

Steps to Reproduce
Using modrate/perf_c2c

Expected results
Reduce contention on entry extension factory

[tbordaz]

RHEL epic 8.6