Filter for controlling what statements are allowed

With this filter, it would be possible to control what kind of statements are allowed. For instance, you could prevent writes, DDL, etc. and even reads, if you wanted to.

One context where this would be useful is when you have one listener + readconnroute to be used for writing and one listener + readconnroute to be used for reading. However, that setup does not prevent someone from opening a connection to the latter listener and proceeding to write. With this filter, it could be ensured that no writes go through.

If you want to use readwritesplit to provide load balanced read access to a cluster, you could setup a service with this filter and be ensured that no writes will go through, not even by a super user.

When the system is ready, you could include this filter in all services to prevent all DDL.