Extra test for region allocate/free

[igchor]

I'm not sure if current region_allocate clears all necessary metadata when taking region from free list.

Pseudocode:

r1 = allocate_region and append some data there
r2 = allocate_region and append some data there

reopen()

free_region(r1)
r3 = allocate_region()

if (testcase1) {
    check if data from r1 is not available
    check timestamps
} else if (testcase 2) {
    append some data to r3
    check if data from r1 is not available
    check timestamps
} else if (testcase3) {
    append some data to r2
    check if data from r1 is not available
    check timestamps
}

The above test cases are only for illustration. I think we could implement those tests as part of stateful testing.