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.

Also make sure to flush pcoinsTip before calling GetStats() to make
sure we apply them to the current height.