LedgerTxn::getHeader is misused

[marta-lokhova]

it looks like getHeader isn't used correctly in the codebase. getHeader is a const getter, and is meant to be used when creating a LedgerTxn child. The problem is that when it comes to modifying the header at apply time, getHeader doesn't check any invariance, like whether there's an active writer already that may modify the header, a child exists (which invalidates the header), or if ledger header is already sealed (which suggests buggy usage). This creates an opportunity to load and use an invalid header. We should switch to using loadHeader instead, which enforces the invariants mentioned.

stellar-core/src/herder/Upgrades.cpp

Line 1317 in e92596f

if (!isLive(ttlLtxe.current(), ltx.getHeader().ledgerSeq))

stellar-core/src/transactions/InvokeHostFunctionOpFrame.cpp

Line 361 in e92596f

if (!isLive(ttlLtxe.current(), ltx.getHeader().ledgerSeq))

stellar-core/src/bucket/BucketManagerImpl.cpp

Line 983 in e92596f

releaseAssert(protocolVersionStartsFrom(ltx.getHeader().ledgerVersion,