feat(server): tighten memory checks when inseting a new object.

[romange]

Before this change Dragonfly evicted items only when it was low on memory and had to grow its main dictionary.
It is not enough because in many cases Dragonfly can grow in memory even when the main dictionary does not grow.
For example, when its dictionary is less than 90% utilized but the newly added objects require lots of memory.

In addition, the dashtable could add additional segments when others had enough available slots to fill the rest of the free memory.

This change adds another layer of defense that allows evicting items even when dictionary segments are not full.
The eviction is still local with respect to a segment. On my tests it seems that it's much harder to cross maxmemory limit than before.

In addition we improved our heuristic that allowed the dashtable to grow. Now it takes into account the average bytes per item in order to
project how much memory the full table takes before adding to it new segments. This really tightens dashtable utilization.

Things to improve:

1. the eviction behavior is rough. Once an insert does the eviction it tries to free enough objects to bring memory back.
   This may result in very spiky insertion/eviction patterns.
2. The eviction procedure, even though it's limited to a single segment, is quite heavy because it goes over all buckets
   in the segment. This may result in weird artifacts where we evict just enough to be under the limit, then add and evict
   again and so on.
3. Therefore, we may need a periodic eviction that will complement this emergency eviction step.

Fixes #224 and partially addresses #256

Signed-off-by: Roman Gershman [email protected]