Fixes for allocator + factory + misc improvements (#3534)

* Clear up import/export misunderstandings

* Fetch minimum period from runtime

* Remove unnecessary comment

This variable is already fetched from the runtime
in the line below.

* Fix bug in factory

The `best_block_id` stayed the same, it was always the
genesis hash. This resulted in the factory failing after
4096 blocks, since `client/db` discards hashes (in this
case the genesis hash) after 4096 blocks from the database.

* Fix tense in error message

* Improve allocator documentation

* Fix bug in allocator

Under certain circumstances an invalid pointer was
returned: when the `ptr` was calculated as equal
to the `max_heap_size`. This is an invalid pointer
since there is no access allowed after the heap limit.

The way to provoke this was to repeatedly allocate
with sizes which were previously not allocated and
immediately deallocate right afterwards. What this
did was to increment the `bumper` with each allocat...