Add adaptive quicklist fill factor

Fill factor now has two options:
  - negative (1-5) for size-based ziplist filling
  - positive for length-based ziplist filling with implicit size cap.

Negative offsets define ziplist size limits of:
  -1: 4k
  -2: 8k
  -3: 16k
  -4: 32k
  -5: 64k

Positive offsets now automatically limit their max size to 8k.  Any
elements larger than 8k will be in individual nodes.

Positive ziplist fill factors will keep adding elements
to a ziplist until one of:
  - ziplist has FILL number of elements
    - or -
  - ziplist grows above our ziplist max size (currently 8k)

When using positive fill factors, if you insert a large
element (over 8k), that element will automatically allocate
an individual quicklist node with one element and no other elements will be
in the same ziplist inside that quicklist node.

When using negative fill factors, elements up to the size
limit can be added to one quicklist node.  If an element
is added larger than the max ziplist size, that element
will be allocated an individual ziplist in a new quicklist node.

Tests also updated to start testing at fill factor -5.
