Each fellow storage reserves a minimum amount of memory.

Closes #51

this wrong line counteracted fdb260e2 :(

Part of the solution to #54

It does not make sense to have it larger than the logbuffer itself.

Because the logbuffer can be larger than the threshold, we need to flush
also for n >= thr.

Fixes 7e986ecc

Motivated by #45

Ref #55