Xray thread pool stales on large-scale Jira instances

Xray thread pool has a maximum of allowed threads equal to the number of CPU cores.

In large-scale intances, this number may be insufficient for the demanding usage of Xray in such environments.

Xray should allow a Jira administrator to increase this value at least to the same number of database connection pools.

Summing up, we should:

• provide a default number of available threads based on the maximum connection pool value (this value should never be higher than the available number of tomcat threads);
• give the possibility for the customers to increase or decrease that value so they can balance it with the usage of Xray on their Jira instances;
• update the documentation with some advice regarding this thread pool.