I misunderstood the purpose of the `heap_index` rdataset header
member; I thought it identified which heap to use, and could therefore
be smaller, the same size as `locknum` indexes. But in fact it is a
position within a heap, so it needs to be able to count up to the
total number of rdatasets in the rbtdb.
So this changes `heap_index` from `uint16_t` back to `unsigned int`.
To avoid re-embiggening the rdatasetheader, shrink the `count` member
from `uint32` to `uint16`. The `count` is used to rotate RRsets in
`dns_rdataset_towiresorted()`, so 16 bits is more than large enough.
This change also means we no longer need to avoid colliding with
`DNS_RDATASET_COUNT_UNDEFINED` i.e. UINT32_MAX.
Closes#3862
9d7b224201