Maintenance release for 1.6.X branch with some fixes backported from recent 1.7.X versions, including a fix for possible connection leak in Hangfire.SqlServer when an exception is thrown while releasing a distributed lock.

Hangfire.Core

Backported from 1.7.19:

• Fixed – Race condition in AspNetShutdownDetector leads to NullReferenceException (by @jr01).

Hangfire.SqlServer

Backported from 1.7.21:

• Fixed – Ensure connection is released when exception is thrown when during lock release.
• Fixed – “A network-related or instance-specific error” when using DisableConcurrentExecution for long-running jobs.