Migrate data to new transactions table

[2opremio]

Also, create data migration infrastructure so that future migrations can be easily added.

[tamirms]

@2opremio overall looks good! Could you add some tests?

[2opremio]

@tamirms Sure thing!

[2opremio]

Uhm, I am starting to think it's a problem with the JSONRPC client.

If I print the listening ports right after the error, I see that soroban RPC is indeed listening on port 8000

    integration.go:265: RPC still unhealthy Post "http://localhost:8000": dial tcp [::1]:8000: connect: connection refused 
[...]
Who is listening on RPC port?
COMMAND    PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
test.test 9885 runner   17u  IPv4 115645      0t0  TCP localhost:8000 (LISTEN)

I think that the client may not retry after the first attempt if it cannot connect :S

[2opremio]

@tamirms This is finally ready to go.

The new migration test required rewriting most of the integration-test infra, so the PR ended up getting quite big (sorry about that).

As a result the integration test infra is way cleaner.

BTW, the spurious test failures I was fighting were due to creachadair/jrpc2#118

[tamirms]

great job debugging creachadair/jrpc2#118 , this looks great!