[ntuple] set NEntries in RPagePersistentSink::InitFromDescriptor

[silverweed]

Also add a bunch of reserve() where appropriate

Checklist:

• tested changes locally
• updated the docs (if necessary)

[jblomer]

LGTM

[github-actions]

Test Results

    18 files      18 suites   4d 10h 15m 38s ⏱️
 2 682 tests  2 680 ✅ 0 💤 2 ❌
46 556 runs  46 553 ✅ 0 💤 3 ❌

For more details on these failures, see this check.

Results for commit 1925f52.

[hahnjo]

(also for the record, I'm getting less sure about all those reserves sprinkled in; at least for me, they really decrease code readability for oftentimes questionable benefit...)

[hahnjo]

Can you comment on why we want this? This now makes InitFromDescriptor different from normal descriptor building where NEntries is only updated when adding a cluster group. This is also documented in the comment of RNTupleDescriptor::fNEntries, which would need at least some updating with this change. More generally, for knowing the number of "entries in flight", we can just look at fPrevClusterNEntries, no?