PERF: fix merging on datetimelike columns to not use object-dtype factorizer
See https://github.com/pandas-dev/pandas/pull/53213#issuecomment-1547352853 for context.

Running the added benchmark on current main gives:

```
[100.00%] ··· join_merge.MergeDatetime.time_merge           ok
[100.00%] ··· ============== ============= =================
              --                            tz              
              -------------- -------------------------------
                  units           None      Europe/Brussels 
              ============== ============= =================
               ('ns', 'ns')    16.1±0.3ms      16.1±0.3ms   
               ('ms', 'ms')   20.7±0.07ms      21.0±0.2ms   
               ('ns', 'ms')     167±2ms        16.9±0.4ms   
              ============== ============= =================

```

versus on this branch:

```
[100.00%] ··· ============== ============= =================
              --                            tz              
              -------------- -------------------------------
                  units           None      Europe/Brussels 
              ============== ============= =================
               ('ns', 'ns')   6.33±0.09ms      6.44±0.1ms   
               ('ms', 'ms')    6.95±0.2ms      7.06±0.1ms   
               ('ns', 'ms')   6.90±0.08ms     6.91±0.09ms   
              ============== ============= =================
```

(and running the standard ns/ns example on pandas 1.5, it also gave around 7ms, so this should restore the performance of 1.5)



- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

@jbrockmendel anything else here, or good to go? Thanks @jorisvandenbossche 