Investigate and fix periodical `put` performance drop

[ikopylov]

Sometimes performance even drops to zero.
In bobp output it looks like this:

put: 76200.00 kb/s | get:   0.00 kb/s | put lat:   0.01 ms | get lat:   0.00 ms
put:   1523 rps  | get      0 rps   | put err:     0     | get err:     0
put: 76150.00 kb/s | get:   0.00 kb/s | put lat:   0.01 ms | get lat:   0.00 ms
put:   1523 rps  | get      0 rps   | put err:     0     | get err:     0
put: 76150.00 kb/s | get:   0.00 kb/s | put lat:   0.01 ms | get lat:   0.00 ms
put:    722 rps  | get      0 rps   | put err:     0     | get err:     0
put: 36100.00 kb/s | get:   0.00 kb/s | put lat:   0.00 ms | get lat:   0.00 ms
put:      0 rps  | get      0 rps   | put err:     0     | get err:     0
put:   0.00 kb/s | get:   0.00 kb/s | put lat:   0.00 ms | get lat:   0.00 ms
put:    116 rps  | get      0 rps   | put err:     0     | get err:     0
put: 5800.00 kb/s | get:   0.00 kb/s | put lat:   0.18 ms | get lat:   0.00 ms
put:    596 rps  | get      0 rps   | put err:     0     | get err:     0
put: 29800.00 kb/s | get:   0.00 kb/s | put lat:   0.01 ms | get lat:   0.00 ms
put:   1524 rps  | get      0 rps   | put err:     0     | get err:     0
put: 76200.00 kb/s | get:   0.00 kb/s | put lat:   0.01 ms | get lat:   0.00 ms
put:   1524 rps  | get      0 rps   | put err:     0     | get err:     0
put: 76200.00 kb/s | get:   0.00 kb/s | put lat:   0.01 ms | get lat:   0.00 ms
put:   1523 rps  | get      0 rps   | put err:     0     | get err:     0

[ikopylov]

This is probably due to fsync call: #748