Update LedgerHeader before applying buckets

[jonjove]

#3233 revealed the version-dependent database input checks can fail during bucket apply. This is fixed in #3235, but not in an elegant way. A better solution would allow us to update the header first. We might already be able to do this, or we might need to use the rebuild-buckets functionality to make this work.

[MonsieurNicolas]

This was fixed in a different way in https://github.com/stellar/stellar-core/pull/3238 that uses the version for the target ledger - still not great to have this temporary override of the lcl.