INT-3398 Polish Javadocs

JIRA: https://jira.spring.io/browse/INT-3398
INT-3399 Update spring-retry to 1.1.0

JIRA: https://jira.spring.io/browse/INT-3399

Fix `MessagingTemplate` javadoc warn
Add the Spring IO Platform plugin

INT-3396: Polishing

JIRA: https://jira.spring.io/browse/INT-3396

Updates for latest snapshots of the Spring IO plugin

Update the configuration to use the renamed plugin. Add a dependency in
the springIoVersions configuration that points to the versions published
by Spring IO.

Upgrade to 0.0.3.RELEASE

Upgrade the plugin version to 0.0.3.RELEASE. Only apply the plugin if
the platformVersion property has been set.

Polishing following upgrade to 0.0.3.RELEASE

- Remove reference to snapshot plugins repository
- Remove unnessary use of the springIoVersions configuration

Improve configuration of snapshot repository

Upgrade to latest SD to be consistent with IO
INT-3407: Fix TX-sync-processor potential NPE

JIRA: https://jira.spring.io/browse/INT-3407