br: make incremental backup and pitr compatible

[BornChanger]

Enhancement

Some BR users want to combine incremental backup and pitr in order to get better RTO and RPO, we need to make them compatible in restricted scenarios.
Sample usage is like this

--Full Restore
./br restore full -s "local:///root/backups/test1/full" -u 127.0.0.1:2379

--Suppose there are N incremental backups, whose backup path names are incremental-*
--And BackupTS of incremental-i is less than that of incremental-j, which i is less than j.

--Incremental Restore incremental-1 ~ incremental-N-1
for i in `seq 1 $((N-1))`; do
    ./br restore full -s "local:///root/backups/test1/incremental-$i" -u 127.0.0.1:2379
done

--Log Restore with Incremental Restore incremental-N as full backup storage
./br restore point -s "local:///root/backups/test1/log" -u 127.0.0.1:2379 --full-backup-storage "local:///root/backups/test1/incremental-$N"