SOLR-16951: Add PKI Auth Caching for both generation and validation

[HoustonPutman]

https://issues.apache.org/jira/browse/SOLR-16951

• Default PKI TTL has been increased to 10 seconds
• The PKI Generation cache will expire cached tokens after 1/4 of the TTL time, to give a minimum 3/4 of the TTL time for transportation.
• The PKI Generation cache will start refreshing cached tokens 1/2 of the expiration time (1/8 of the TTL time), to give time for the asynchronous refresh to complete before the token is expired. I would set this to expirationTime - 50ms, but CaffeineCache requires a get() in order to trigger the async refresh, so we want to give time between refreshTime and expireTime for a request to come in to trigger the refresh. 1/2 of the expiration time should be fine here.
• I don't expect to get more than 100 users that often, so I think this is a safe cache limit for generation. For validation, the node name is included in the token, so we need an order of magnitude bigger cache. I chose 1000 here, but this can probably be changed if we need to. The validation cache isn't as important as the generation cache, so it shouldn't be a huge deal.