INT-4212: FWMH Predicates and DSL: flushWhenIdle

JIRA: https://jira.spring.io/browse/INT-4212

Add `firstWrite` to predicate methods.
Add DSL support for `flushWhenIdle`

While these are breaking API changes on the interfaces, I feel this is
likely not going to affect many users and this is a major release after
all.

I have added a note to the 5.0 migration guide.

src/reference/asciidoc/file.adoc

@@ -660,6 +660,7 @@ or `FileWritingMessageHandler.MessageFlushPredicate` implementation.
 
 The predicates are called for each open file.
 See the java docs for these interfaces for more information.
+Note that, since _version 5.0_, the predicate methods provide another parameter - the time that the current file was first written to if new or previously closed.
 
 When using `flushInterval`, the interval starts at the last write - the file is flushed only if it is idle for the interval.
 Starting with _version 4.3.7_, and additional property `flushWhenIdle` can be set to `false`, meaning that the interval starts with the first write to a previously flushed (or new) file.

src/reference/asciidoc/whats-new.adoc

@@ -61,6 +61,9 @@ See <<file-reading>> for more information.
 The tail adapter now supports `idleEventInterval` to emit events when there is no data in the file during that period.
 See <<file-tailing>> for more information.
 
+The flush predicates for the `FileWritingMessageHandler` now have an additional parameter.
+See <<file-flushing>> for more information.
+
 ==== (S)FTP Changes
 
 The inbound channel adapters now have a property `max-fetch-size` which is used to limit the number of files fetched during a poll when there are no files currently in the local directory.