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.
GetStats is a very slow call (it iterates over the entire database), so the overhead of an extra flush is negligible in comparison.