[Improvement: ZSTACK-40364] refactor fencer for SBLK
[Improvement: ZSTACK-40364] refactor fencer for SBLK

Prior to this patch, each SBLK connected to current host consumes 1
thread in kvmagent.  This is OK for most setups but not for when the
number of SBLK grows to a larger number, due to two main causes:

1. The overhead of GIL becomes significant;
2. Repetitive and excessive calls to shell make things even worse.

The check against 'next_errors' in renewal history is skipped.

Signed-off-by: Qun Li <1ea81f54854048d0a4d4fd61caceabb2a843281c@zstack.io>

zstackio/zstack-utility,zstackio/zstack-utility,zstackio/zstack-utility