- restructure sections
- improve readability

[FLINK-3825] Documentation for CEP Scala API. Minor beautification on (flat-)select function code example.

This closes #1960.

This closes #1964.

Per timestamp only one TriggerTask is registered at
the runtime context. When the first timer is registered a new TriggerTask
is sheduled. When no timer is registered anymore for a specific timestamp
the corresponding trigger task is canceled and hence removed.

The ScheduledFutures to cancel trigger tasks are not checkpointed. So
cleanup of trigger tasks will not work after a failure.

This closes #1955

Vertex sets are now merged with a cogroup so that a new Vertex is only
added to the Graph when no existing Vertex exists with the same label.

This closes #1949

Methods for translation of the type or value of graph IDs, vertex
values, and edge values.

This closes #1900

This closes #1943

This closes #1928.

This closes #1950

This closes #1951

All original edges are now emitted when no matching edge is found in the
edge removal set.

This closes #1948

This closes #1926.

Closed because CsvTableSource has been implemented.
This closes #939.

This closes #1764

We create random files on initialization to check whether we can write
to the data directory. These files are also removed now.

- Add CsvTableSource as a reference implementation for TableSources.

This closes #1939

When the ResourceManager receives a new leading JobManager via the
LeaderRetrievalService it tries to register with this JobManager until
connected. If during registration a new leader gets elected, the
ResourceManager may still repeatedly try to register with the old
one. This doesn't affect the registration with the new JobManager but
leaves error messages in the log file and may process unnecessary
messages.

This closes #1912.

This closes #1933

This closes #1932

This closes #1936

This closes #1937

This closes #1934

This PR terminates the ExecutionGraphs properly without restarts when the JobManager calls
cancelAndClearEverything. It is achieved by allowing the method to be only called with an
SuppressRestartsException. The SuppressRestartsException will disable the restart strategy of
the respective ExecutionGraph. This is important because the root cause could be a different
exception. In order to avoid race conditions, the restart strategy has to be checked twice
whether it allwos to restart the job: Once before and once after the job has transitioned to
the state RESTARTING. This avoids that ExecutionGraphs can become an orphan.

Furhtermore, this PR fixes the problem that the default restart strategy is shared by multiple
jobs. The problem is solved by introducing a RestartStrategyFactory which creates for every
job its own instance of a RestartStrategy.

Fix LeaderChangeJobRecoveryTest case

This closes #1923.

Added missing test dependency to pom.

Pattern in Scala API now uses Option to shield users against null values from the Java API

Added test-jar for build phase to pom.

Removed necessary dependency entry and an overlooked comment.

Added missing comments on FollowedByPattern.

CEPIT test update.

Replace CEP test with a simple test for Scala <-> Java interoperability.

This closes #1905.

This closes #1903

This closes #1922

This closes #1935.