Replace assert statements with explicit checks

[shoothzj]

Main Issue #995

Motivation

In the current implementation, several components use assert statements to verify conditions. However, assert statements are not typically enabled in production environments, which means these checks can be bypassed, potentially leading to unclear errors later in the execution. Replacing these with explicit checks ensures that our error handling is active and clear during production runs.

Changes

• BKLogSegmentWriter.java: Added a check to verify the context type before casting to BKTransmitPacket.
• BKDLConfig.java: Introduced a check for the type of dlConfig before casting to BKDLConfig.
• ZKVersionedSetOp.java: Added a check to ensure the operation result is of type OpResult.ErrorResult.