Support read-only modes for MPT tools

[HerbertJordan]

To support cases where data bases are protected by OS level permissions, verification tools should be able to work on files where there are only read permissions.

Tools to be included:

• the info tool
• the export tool
• the verification tool

[thaarok]

For checking of the consistence of Carmen database with go-opera leveldb database, I still need to open the database to obtain:

• live state hash
• last archive block hash

As long it is not possible to obtain it without opening the state for writing, issues like #679 can occur (the database was corrupted, because the process was terminated in the middle of unnecessary state flushing)