Deprecate AbstractLogstashTcpSocketAppender#secondaryConnectionTTL

[brenuart]

Property secondaryConnectionTTL belongs to the PreferPrimaryDestinationConnectionStrategy connection strategy and should not be exposed by the AbstractLogstashTcpSocketAppender.

This property has been kept in AbstractLogstashTcpSocketAppender for backwards compatibility purposes from before the connection strategies were implemented in #195 in 4.10. Previously (< 4.10) the concept of a connection strategy did not exist, and secondaryConnectionTTL was a just property on the appender. In 4.10, connection strategies were implemented, and the property remained on the appender for backwards compatibility.

As described in the documentation, the recommended way to configure this property is by setting it directly on the preferPrimary connection strategy as illustrated in the following example:

      <connectionStrategy>
          <preferPrimary>
              <secondaryConnectionTTL>5 minutes</secondaryConnectionTTL>
          </preferPrimary>
      </connectionStrategy>

The property should be deprecated in a backward compatible way and marked for removal in the next major release (8.0).

(see #576 (comment) for more background information)