Fixed new bshrub roots and related bug fixing

It turned out by implicitly handling root allocation in
lfsr_btree_commit_, we were never allowing lfsr_bshrub_commit to
intercept new roots as new bshrubs. Fixing this required moving the
root allocation logic up into lfsr_btree_commit.

This resulted in quite a bit of small bug fixing because it turns out if
you can never create non-inlined bshrubs you never test non-inlined
bshrubs:

- Our previous rbyd.weight == btree.weight check for if we've reached
  the root no longer works, changed to an explicit check that the blocks
  match. Fortunately, now that new roots set trunk=0 new roots are no
  longer a problematic case.

- We need to only evict when we calculate an accurate estimate, the
  previous code had a bug where eviction occurred early based only on the
  progged-since-last-estimate.

- We need to manually set bshrub.block=mdir.block on new bshrubs,
  otherwise the lfsr_bshrub_isbshrub check fails in mdir commit staging.

Also updated btree/bshrub following code in the dbg scripts, which
mostly meant making them accept both BRANCH and SHRUBBRANCH tags as
btree/bshrub branches. Conveniently very little code needs to change
to extend btree read operations to support bshrubs.

scripts/dbgmtree.py

@@ -596,7 +596,7 @@ class Rbyd:
                     rid_, w = rid__, w_
 
                 # catch any branches
-                if tag == TAG_BRANCH:
+                if tag & 0xfff == TAG_BRANCH:
                     branch = (tag, j, d, data)
 
                 tags.append((tag, j, d, data))
@@ -703,7 +703,10 @@ class Rbyd:
                     ))
 
                 d_ += max(bdepths.get(d, 0), 1)
-                leaf = (bid-(w-1), d, rid-(w-1), TAG_BRANCH)
+                leaf = (bid-(w-1), d, rid-(w-1),
+                    next((tag for tag, _, _, _ in tags
+                            if tag & 0xfff == TAG_BRANCH),
+                        TAG_BRANCH))
 
         # remap branches to leaves if we aren't showing inner branches
         if not inner: