db: close Berkeley DB cursors instead of leaving them to the handle (#71)

Three functions open a cursor and none of them close it: CTxDB::LoadBlockIndex,
CTxDB::ReadOwnerTxes and CWalletDB::LoadWallet.

Berkeley DB requires every cursor to be closed before the handle it came
from. ~CDB() calls Close(), which calls pdb->close() inside a catch-all
that swallows whatever it returns -- so the violation was real and silent,
which is this codebase's favourite kind.

An open cursor also holds read locks, and that is not theoretical here: the
first version of the startup chain repair (#61) opened a write transaction
beside the cursor LoadBlockIndex was still holding and waited on it
forever, coming up, printing its diagnosis and hanging with nothing in any
log. The repair had to be moved to the caller to get clear of it.

A scope guard rather than a close call at each return, because all three
functions have several exits and the point is to stop relying on anyone
remembering. Same fix Bitcoin made in c5c7911da.

Verified: node starts, verifies 288 blocks, loads the wallet, runs the
spent-flag rescan and syncs the live chain to 4422 with no Berkeley DB
error anywhere and db.log still empty.

0 files changed, 0 insertions, 0 deletions