emit warning if bl merge slower than 50 seconds

[ThomasBrady]

Description

Resolves https://github.com/stellar/stellar-core-internal/issues/275

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