(WIP) add cluster meta executor

[dgnorton]

This PR adds cluster.MetaExecutor which distributes InfluxQL meta statements to all data nodes in the cluster for execution. For example, when DROP DATABASE foo is executed, it goes through the following flow:

• HTTP API receives query
• HTTP API passes query to cluster.QueryExecutor
• cluster.QueryExecutor tells meta.Client to delete the database
• cluster.QueryExecutor tells cluster.MetaExecutor to execute the statement
• cluster.MetaExecutor creates an executeStatementRequestMessage RPC command which is sent to cluster.Service on all data nodes
• cluster.Service dispatches command to cluster.Service.processExecuteStatementRequest
• cluster.Service.processExecuteStatementRequest unmarshals the request, parses the statement, and passes it to cluster.Service.executeStatement
• cluster.Service.executeStatement switches on the statement type and calls cluster.MetaWriter.DropDatabase
• cluster.MetaWriter.DropDatabase calls tsdb.Store.DeleteDatabase

Errors propagate back from each node through RPC to the node that executed the statement on the cluster and that node returns failure if any node fails to execute.

[e-dard]

Closing in favour of #5753.