RE: speed up a logical replica setup

Dear hackers,

Based on the requirement, I have profiled the performance test. It showed bottlenecks
are in small-data case are mainly two - starting a server and waiting until the
recovery is done.

# Tested source code

V7 patch set was applied atop HEAD(0eb23285). No configure options were specified
when it was built.

# Tested workload

I focused on only 100MB/1GB cases because bigger ones have already had good performance.
(Number of inserted tuples were same as previous tests)
I used bash script instead of tap test framework. See attached. Executed SQLs and
operations were almost the same.

As you can see, I tested only one-db case. Results may be changed if the number
of databases were changed.

# Measurement
Some debug logs which output current time were added (please see diff file).
I picked up some events and done at before/after them. Below bullets showed the measured ones:

* Starting a server
* Stopping a server
* Creating replication slots
* Creating publications
* Waiting until the recovery ended
* Creating subscriptions

# Result
Below table shows the elapsed time for these events. Raw data is also available
by the attached excel file.

|Event category |100MB case [sec]|1GB [sec]|
|Starting a server |1.414 |1.417 |
|Stoping a server |0.506 |0.506 |
|Creating replication slots |0.005 |0.007 |
|Creating publications |0.001 |0.002 |
|Waiting until the recovery ended|1.603 |14.529 |
|Creating subscriptions |0.012 |0.012 |
|Total |3.541 |16.473 |
|actual time |4.37 |17.271 |

As you can see, starting servers and waiting seem slow. We cannot omit these,
but setting smaller shared_buffers will reduce the start time. One approach is
to overwrite the GUC to smaller value, but I think we cannot determine the
appropriate value.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED