G1: Refactor G1BlockOffsetTable::verify

Just like the `SerialBlockOffsetTable::verify_for_block` and `ObjectStartArray::verify_for_block`, the `G1BlockOffsetTable::verify` should be invoked after each time writing. And its name `verify`, signature and feature should be adjusted to keep consistence.

This issue was reported during reviewing JDK-8329603.

[1] https://github.com/openjdk/jdk/pull/18634#discussion_r1553227054