Allow user to disable readahead by setting max-readahead smaller than blocksize

[polyrabbit]

Allow user to disable read-ahead when he is doing random read - to reduce read amplification and improve read latency.

[jiefenghuang]

Perhaps a more explicit flag would be better, such as -1 to disable readahead.
related: #5224

[polyrabbit]

I suppose it's better to measure max-readahead in blocks, not MiB, in consistent with the code logic.

Current value of max-readahead results in inconsistent readahead blocks:

max-readahead value	readahead blocks
[blockSize, +inf)	max-readahead/blockSize
(0, blockSize)	0
[0]	8