[ROCKETMQ-270] Move flush position forward to first MappedFile whose start offset is non-zero

[zhouxinyu]

What is the purpose of the change

To resolve the issue ROCKETMQ-270.

Slave Broker can't flush data to disk normally if the master broker has been cleaned commit log.

Brief changelog

When the flush position is zero and the first MappedFile's start offset is non-zero, move it forward to the first MappedFile's start offset. Otherwise, slave broker can't flush data to disk normally.

Verifying this change

1. Start master broker, and write lots of messages until broker starts cleaning commit log.
2. When the first commit log file isn't 00000000000000, start slave broker.
3. Slave broker now can flush data normally.

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[vongosling]

LGTM

[coveralls]

Coverage decreased (-0.1%) to 39.118% when pulling f18dc03 on zhouxinyu:ROCKETMQ-270 into 1a88fe0 on apache:develop.