Improve node startup time

[terror]

We have many tests to run, node startup was really slow, these changes speed it up:

• Move sleep to end of client creation loop and reduce it to 100ms
• Don't create wallet by default

[RCasatta]

It looks versions < 0.21 don't support this... And also I am afraid not creating the default wallet will break too much users code:

I would suggest:

• A standalone PR with only the non-controversial "Move sleep to end of client creation loop and reduce it to 100ms" which I would merge right away
• Another PR with a new configuration parameter like skip-default-wallet-creation, which is false by default so it keeps current behavior but that you can use in your tests to speed things up

[terror]

It looks versions < 0.21 don't support this... And also I am afraid not creating the default wallet will break too much users code:

I would suggest:

• A standalone PR with only the non-controversial "Move sleep to end of client creation loop and reduce it to 100ms" which I would merge right away
• Another PR with a new configuration parameter like skip-default-wallet-creation, which is false by default so it keeps current behavior but that you can use in your tests to speed things up

I see, I didn't realize versions < 0.21 didn't support this. I'll modify this PR to include the changes you suggested!

[RCasatta]

There are still some issue in the CI, note I created a separated MR #83 for just the sleep thing (I think I am going to release a version soon, so I thought to include the easy part in the release)