blob: fix append of non-exisient entries with EXTHDR

Now we threat appends of non-existent entries as simple writes
blob: prealloc space on append

Write of non-exisient entry with APPEND flag set is a very strong
indication that this entry will be appended, so preallocate it.
tests: stress: added test for non-existient entry append
tests: stress: overwrite w/o csum
blob: removed binlog

NB! Now data-sort now bails out at start.