Include ledger sequence in preflight response

[tamirms]

Description

This commit adds a ledger sequence to the preflight response which is useful because execution of a smart contract will depend on the state of the ledger when the operation is applied.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[paulbellamy]

lgtm, but idk

[MonsieurNicolas]

Any reason for moving ltx out of the try/catch block? The constructor call throw and this would crash the node instead of returning an error

[tamirms]

I assumed that if the invoke function operation failed (e.g. the smart contract reverted) then that would throw an exception and we would enter the catch clause. In that casem it would still be nice to know the ledger sequence of the invoke function attempt.

I assumed that moving ltx out of the try/catch block should be safe because we already do that in the get ledger entry endpoint:

stellar-core/src/main/CommandHandler.cpp

Lines 722 to 723 in e7f06ea

	LedgerTxn ltx(mApp.getLedgerTxnRoot());
	root["ledger"] = ltx.loadHeader().current().ledgerSeq;

Note how the same code is executed without any try / catch block in the handler.

[MonsieurNicolas]

Actually commandhandler has a try/catch anyways at the top level (just returns a 500) so we're good

[MonsieurNicolas]

r+ e7f06ea