publish: build checkpoint files during ledger close

[marta-lokhova]

Resolves #4212

The change removes publish dependency on txhistory and ledgerheaders SQL tables, and instead constructs ledger headers/tx sets/tx results checkpoints at ledger close. This allows us to deprecate and drop support for txhistory entirely, which is beneficial for performance. Additionally, the code became simpler, and the xdr->sql->xdr roundtrip processing has been removed (as part of this, txsethistory table was dropped as well). The tradeoff in this approach is that we lose atomicity we had with SQL transactions. As a result, core now manually has to handle crashes. This is done by always constructing "tmp" checkpoints first, and only promoting them after a checkpoint ledger has been committed to the database. In the event that tmp file has been written, but core crashed in apply and rolled back the transaction, there's automatic file cleanup on startup, that trims any uncommitted ledgers from the temp file.

[marta-lokhova]

Note: the current version of this PR drops txhistory table for simplicity, however depending on who depends on it, we might need to still support it, while also building checkpoint files directly.