saw many logs of "Imported new potential chain segment", may impact performance

[zzzckck]

System information

Geth version: geth v1.4.11
OS & Version: Linux
Commit hash : tag v1.4.11

Expected behaviour

"Imported new potential chain segment" is mainly caused by off-turn validator produce off-turn block, as it failed to received the in-turn block on time.
Too many of this log may impact the performance, did some analyze, it seems the log was printed more frequently since mid of May. Recently, there are around 800 logs per day.

Actual behaviour

should have less such kind of log.

Steps to reproduce the behaviour

NA

Backtrace

NA

[zzzckck]

maybe we can increase the 1st off-turn block backoff from 1s to 2s

[zzzckck]

it is not a new issue, actually this PR change the log and expose the issue since v1.4.4, #2352