Support secondary log directory

[tabokie]

It can be useful to configure raft engine on a separate (smaller) disk. And when that disk is full, we should provide a way for raft-engine to locate newly created log files on the main disk volume.

[LykxSassinator]

U mean that we need a new configuration such as sub-dir or assist-dir to store logs on different disk?

With this pre-requisite, the main dir, that is, the current configuration dir, is still used as the default main directory to store logs. Only if the main dir is FULL or not accessible, we will use the assist-dir or sub-dir to store the incoming logs.

Meanwhile, about the abnormal circumstance:

• sub-dir is None or belongs to the same volumn of disk, if the disk is full, the newly file creation should fail.

[LykxSassinator]

/assign @LykxSassinator Let me tackle it.