Implement token storage solution based on MySQL

Hadoop RBF supports custom implementations of secret managers. At the moment, the only available implementation is ZKDelegationTokenSecretManagerImpl, which stores tokens and delegation keys in Zookeeper.

During our investigation, we found that the performance of routers is limited by the writes to the Zookeeper token store, which impacts requests for token creation, renewal and cancellation. An alternative secret manager implementation has been created, based on MySQL, to handle a higher number of writes.

We measured the throughput of each token operation (create/renew/cancel) on different setups and obtained the following results:

1. Sending requests directly to Namenode (no RBF):
   Token creations: 290 reqs per sec
   Token renewals: 86 reqs per sec
   Token cancellations: 97 reqs per sec

1. Sending requests to routers using Zookeeper based secret manager:
   Token creations: 31 reqs per sec
   Token renewals: 29 reqs per sec
   Token cancellations: 40 reqs per sec
2. Sending requests to routers using SQL based secret manager:
   Token creations: 241 reqs per sec
   Token renewals: 103 reqs per sec
   Token cancellations: 114 reqs per sec

We noticed a significant improvement when using a SQL secret manager, comparable to the throughput offered by Namenodes.