Add a new WT_SESSION::transaction_range_pin method.

Can be used by applications to identify sessions that are keeping
transaction IDs pinned.
That wasn't right either :(

How about this:

```c
uint64_t pinned = txn_state->snap_min;
if (pinned == WT_TXN_NONE ||
    (txn_state->id != WT_TXN_NONE && TXNID_LT(txn_state->id, pinned)))
        pinned = txn_state->id;
if (pinned == WT_TXN_NONE)
        *prange = 0;
else
        *prange = txn_global->current - pinned;
```