rpc: Fix leveldb iterator leak, and flush before `gettxoutsetinfo`

This fixes an iterator leak resulting in

    bitcoind: db/version_set.cc:789: leveldb::VersionSet::~VersionSet(): Assertion `dummy_versions_.next_ == &dummy_versions_' failed."

exception on shutdown (as well as keeping a copy of the utxo data pinned on disk until next restart).

Also make sure to flush pcoinsTip before calling GetStats() to make sure we apply them to the current height.
No controversy, just pointing out that it doesn't do what may be expected. A quick comment would be enough to shut me up :)