rewrite alloc to be able to check availability of memory

[lll-phill-lll]

Here is memory consumption and limit of WideCombiner measured in kqprun with default settings.

The limit is always close to the memory consumption. And therefore we cannot predict whether we will have enough memory for the next x2 growth based only on the limit. We need to add some methods to the allocator so that we can know in advance whether we can survive a sharp growth of the hash table.