common: add requirement for unlimited virtual memory

Some tests fail if overcommit is disabled and virtual
memory is limited due to large memory requirements.
Such tests can require both overcommit enabled and
unlimited virtual memory.
blk: reduce the minimum BTT size to 16MB

Reduce the minimum BTT size to 16MB
and the libpmemblk minimum size to 16MB+8KB.
Update two unit tests.
Merge pull request #88 from ldorau/blk-reduce-the-minimum-BTT-size-to-16MB

blk: reduce the minimum BTT size to 16MB
Merge pull request #87 from plebioda/fix-overcommit-memory-issue

common: add requirement for overcommit memory in unittests