readahead: make context readahead more conservative

This helps performance on moderately dense random reads on
SSD.

Transaction-Per-Second numbers provided by Taobao:

  QPS case

-------------------------------------------------------
  7536 disable
context readahead totally
w/ patch: 7129 slower size rampup and start RA
on the 3rd read
  6717 slower size rampup
w/o patch: 5581 unmodified
context readahead

Before, readahead will be started whenever reading
page N+1 when it happen
to read N recently.  After patch, we'll only
start readahead when *three*
random reads happen to access pages N, N+1,
N+2.  The probability of this
happening is extremely low for pure random
reads, unless they are very
dense, which actually deserves some
readahead.

Also start with a smaller readahead window.  The impact to
interleaved
sequential reads should be small, because for a long run
stream, the the
small readahead window rampup phase is negletable.

The
context readahead actually benefits clustered random reads on HDD
whose
seek cost is pretty high.  However as SSD is increasingly used
for
random read workloads it's better for the context readahead to
concentrate
on interleaved sequential reads.

Another SSD rand read test
from Miao

        # file size:        2GB
        # read IO amount:
625MB
        sysbench --test=fileio          \

--max-requests=10000    \
                --num-threads=1         \

--file-num=1            \
                --file-block-size=64K   \

--file-test-mode=rndrd  \
                --file-fsync-freq=0     \

--file-fsync-end=off    run

shows the performance of btrfs grows up
from 69MB/s to 121MB/s, ext4 from
104MB/s to
121MB/s.

https://github.com/flar2/flo/commit/38ae5ff3b26659cd62fe2e63012518f7b08b5880