[Feature] Sequential migration execution for try-runtime

[ruseinov]

Fixes https://github.com/paritytech/substrate/issues/12295

@bkchr let me know if that's what you had in mind.

[ruseinov]

Logic def. sounds good, but not sure what is going on in the tests, have to look later.

Just removed the redundant calls to pre/post. Because all of this is happening in on_runtime_upgrade now and the state is being passed from one to another there too, sequentially.

[ruseinov]

bot rebase

[paritytech-processbot[bot]]

Rebased

[ruseinov]

bot merge

[paritytech-processbot[bot]]

Waiting for commit status.