Implement an API to return Solr and GC logs

Solr shows the most recent log messages in the UI today. But it is kept in memory and the number of events returned is limited. It is very useful to retrieve the real log from servers during troubleshooting sessions. In most places, without involving security issues, human intervention is required to login, locate and download the log files which slows down the troubleshooting process.

I propose to introduce a logging API (perhaps we just enhance the existing /admin/logging handler) which can stream logs before/after/between given timestamp(s), filter by level etc to help debug issues faster. Similar to Solr logs, the same API can also output GC logs as well. We can choose to keep this API disabled by default for security or keep it enabled but call it out in the Upgrading section of CHANGES.txt