Minimum HashTable size (number of slots) should be dynamically configurable

We have some customers which want to avoid the slowness experienced during HashTable resize at all cost, as their dataset grows and shrinks.

They could set their minimum HashTable size to a large enough size to avoid resizing occurring, at the expense of higher memory usage. To choose the correct minimum HashTable size, they can use the maximum size of any HashTable (vBucket), provided by kv_vb_ht_max_size.

Currently the minimum size is controlled by the ht_size config, which is not dynamically configurable, requiring bucket restart to be applied.

This is a short term work-around until an improved resizing algorithm is deployed.