[BEAM-10249] Populate state cache with initial values before appending

[mxm]

When new values are added to a bag state, the cache handler in the Python SDK
does not check whether the underlying state already has values present. Although
the append operation will be sent to the backend, the cache itself will be
corrupted because it returns only the newly appended values for get operations,
not the values present before the append operation.

This is a concern when restoring from a checkpoint but also when items are
evicted from the cache.

Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Samza	Spark
Go		---	---		---
Java
Python		---			---
XLang	---	---	---		---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[mxm]

Run Python2_PVR_Flink PreCommit

[mxm]

Failures are unrelated to this change.

12:15:27     import rsa
12:15:27   File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python2_PVR_Flink_Phrase/src/build/gradleenv/1866363813/local/lib/python2.7/site-packages/rsa/__init__.py", line 24, in <module>
12:15:27     from rsa.key import newkeys, PrivateKey, PublicKey
12:15:27   File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python2_PVR_Flink_Phrase/src/build/gradleenv/1866363813/local/lib/python2.7/site-packages/rsa/key.py", line 1
12:15:27 SyntaxError: Non-ASCII character '\xc3' in file /home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python2_PVR_Flink_Phrase/src/build/gradleenv/1866363813/local/lib/python2.7/site-packages/rsa/key.py on line 1, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details

[mxm]

Also Dataflow tests are failing (unrelated):

12:46:05 1: Task failed with an exception.
12:46:05 -----------
12:46:05 * Where:
12:46:05 Script '/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/dataflow/common.gradle' line: 77
12:46:05 
12:46:05 * What went wrong:
12:46:05 Execution failed for task ':sdks:python:test-suites:dataflow:py2:preCommitIT_batch'.
12:46:05 > Process 'command 'sh'' finished with non-zero exit value 1
12:46:05 
12:46:05 * Try:
12:46:05 Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
12:46:05 ==============================================================================
12:46:05 
12:46:05 2: Task failed with an exception.
12:46:05 -----------
12:46:05 * Where:
12:46:05 Script '/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/dataflow/common.gradle' line: 77
12:46:05 
12:46:05 * What went wrong:
12:46:05 Execution failed for task ':sdks:python:test-suites:dataflow:py2:preCommitIT_streaming'.
12:46:05 > Process 'command 'sh'' finished with non-zero exit value 1
12:46:05 
12:46:05 * Try:
12:46:05 Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
12:46:05 ==============================================================================
12:46:05 
12:46:05 3: Task failed with an exception.
12:46:05 -----------
12:46:05 * Where:
12:46:05 Script '/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit/src/sdks/python/test-suites/dataflow/common.gradle' line: 77
12:46:05 
12:46:05 * What went wrong:
12:46:05 Execution failed for task ':sdks:python:test-suites:dataflow:py2:preCommitIT_batch_V2'.
12:46:05 > Process 'command 'sh'' finished with non-zero exit value 1
12:46:05

[mxm]

Run Python2_PVR_Flink PreCommit

[mxm]

Run Portable_Python PreCommit

[mxm]

Run Python PreCommit