[5.0] Fix chainbase issue when shrinking the database size.

[greg7mdp]

Along with the changes for map_private mode, an issue sneaked in that Matt identified in his testing.

When the user tries to shrink the chainbase db (i.e. sets chain-state-db-size-mb to a lower number) and restarts nodeos, we internally keep track of a wrong database_size, which leads to possible crashes.