txnkv: fix the issue that deleteRange cannot use nil as endkey (tikv#429

)

Signed-off-by: disksing <i@disksing.com>

txnkv: decrease some max and base backoff time (tikv#388)


Signed-off-by: youjiali1995 <zlwgx1023@gmail.com>

branch name related changes (tikv#146)

Signed-off-by: disksing <i@disksing.com>

Update comment for Scan (tikv#75)

Signed-off-by: lucklove <gnu.crazier@gmail.com>