liblp: Try to merge extents during partition resizes.

When adding extents to partitions, if the previous extent and new extent
are contiguous, merge them together to avoid allocating unnecessary
device-mapper targets.

Bug: 79173901
Test: liblp_test gtest
Change-Id: I80087df9aea8141c5e16f8d4cdb3dd7da02aee8c