reshard: limiting the number of log to be recorded

reshard: limiting the number of log to be recorded

When the bucket's index shards are already overloaded,  avoid
adding too many extra keys in the reshard log. Limiting the size
of this reshard log to `rgw_reshardlog_threshold`, if an index
write operation during the logrecord stage would exceed that limit,
returning the ERR_BUSY_RESHARDING error early.

Using the reshardlog_entries in `rgw_bucket_dir_header` to do this,
when writting shards, adding the reshardlog_entries. But not need
to add in deleting, because number of index entries reduce meanwhile.

Signed-off-by: Mingyuan Liang <liangmingyuan@baidu.com>