Add test for prepared transactions.

[hlinnaka]

I think the handling of prepared transactions is broken in the page
server, so this is not passing.

[hlinnaka]

@knizhnik, I was trying to understand the code you added to handle pg_twophase, and wrote the attached test case to see how it works. It doesn't. I guess that's a known issue?

If it's not working, can we remove all the half-baked code that tries to deal with it, maybe put it in a branch or something? I was trying to rebase #79, and it's hard to refactor things when I don't know what's supposed to work and what's not.

[knizhnik]

Sorry, 2pc support was not completed yet. I have made some fixes but have not committed them yet.
I am now trying complete approach with starting compute node without sending and replaying WAL to it.
All tests are passed except test_branch_behind. But this is actually the only test where postgres is working with datadir created from page store. So I am in process of debugging.

[hlinnaka]

I pushed PR #173, so this is passing now. I'll let the CI run through it one last time, and will push.