When mapped_private doesn't work, database is left dirty

[eosusa]

When I tried to flip a server running 5.0 to the new "mapped_private" memory mode, I encountered the following error (because my chain state db setting was higher than mem on the server):

rethrow Cannot allocate memory:
    {"what":"Cannot allocate memory"}
    nodeos  chain_plugin.cpp:1102 plugin_initialize

The problem is that when i flipped the mode back to the default "mapped" mode, it left the state dirty and I had to use a snapshot to recover it (which Kevin said is not the expected behavior).

[greg7mdp]

This is an easy fix imo. When in mapped_private mode, I don't believe we need to mark the database dirty at startup, since we don't change it until exit.