HAMMER 33/many: Expand transaction processing, fix bug in B-Tree

HAMMER 33/many: Expand transaction processing, fix bug in B-Tree

* Expand transaction processing to cover more of the code space for
  upcoming undo code.

* Fix a bug in the btree_split_leaf(), the separator would sometimes not
  properly be to the left of the split point, resulting in a panic.
  Temporarily add many more assertions to btree_split_leaf().

* Improve the critical path for blockmap lookups, the (newly) passed trans
  already contains a referenced root volume so the blockmap code does not
  have to acquire one.

Reported-by: YONETANI Tomokazu <qhwt+dfly@les.ath.cx> (B-Tree bug)