Release v0.20.1-incubating-rc1

Update pom template for Heron artifacts (#3182)
Add ci script for building docker image (#3181)
[Streamlet] Fix streamlet config to apply cpu and ram in topology (#3180)
Soften padding constraint (#3178)
Add log line to avoid ambiguity (#3177)
Add build scripts for ci jobs (#3174)
Another memory leak in stmgr-client (#3176)
Fix memory leak in stmgr when measuring tuple data size (#3175)
Add simulator maven pom file (#3171)
Add target to generate packages for heron simulator (#3163)
Joshfischer/aws helm fix (#3168)
fix all example topologies (#3167)
create a symbolic link for kubernetes conf after loading docker image (#3161)
remove heron-core.tar.gz from docker image (#3164)
keep executor running when heron update adds containers (#3162)
Support instance healthcheck api (#3157)
Yaoli/fix dockerfiles (#3160)
Fix docker build artifacts script to have the correct version string (#3156)
Add tooltip for component connections (#3155)
Fix RoundRobinPacking repack with no specified numContainers (#3152)
Clean up metrics section in Heron UI (#3153)
Validate resource constraint (RAM and CPU) in RoundRobinPacking (#3142)
Update topology UI to make topology info more consistent across pages (#3151)
Add a release-yaml-file argument to CLI (#3150)
Clean up a url format code (#3149)
Document new streamlet operations in Heron website (#3148)
[UI] Move reset view button to left and change plan graph listener from click to dblclick (#3147)
remove .sh files from docker image (#3146)
Refactor KVStreamlet to make it lighter and easier to convert from Streamlet (#3135)
Update backpressure explanation in Heron architecture doc (#3145)
Add a new streamlet integration test with keyBy, countByKey and … (#3140)
Clean up heron-client-install.sh (#3141)
Add simulator into release package, and explicitly add java/scala api (#3144)
Fix integration test script on mac (#3143)
Nwang/add byte received metric (#3136)
Update integration test runner to support language and test selection (#3139)
typo (#3138)
Refactor StreamletShadow class to be more general (#3134)
Add sum/min/max reducers (#3132)
Nwang/add count and reduce operations in streamlet (#3131)
Add KVStreamlet and keyBy() operation (#3125)
ByteAmount String Format (#3129)
Fix docker rc and scripts for building heron (#3130)
Nwang/fix streamwindowoperator generic types (#3124)
Add CumulativeCountMetric and metrics to monitor instance task and metrics collection (#3127)
Add favicon (#3128)
Refactor window operations to avoid WindowConfigImpl cast (#3123)
Add util function for registerTopologyTimerEvents in Config (#3126)
[Scala Streamlet API] Add Integration Test for Source and Union Operations (#3121)
Add withStream() in Streamlet to support stream selection (#3109)
[Java Streamlet API] Support Abstractions on Streamlet Sources (#3118)
Support utils.Time of kafka-client-spout (#3116)
Nwang/add streamlet operator interfaces with grouping v2 (#3099)
[Java Streamlet API] Move Source Logics to Builder (#3115)
Enable Streamlet Scala API Documentation (#3114)
[Java Streamlet API] Support abstractions on Streamlet Operators (#3112)
Update mailing list informtion in contributing.md (#3110)
[Java Streamlet API] Extend Validations Part II (#3111)
Nwang/add spout based source streamlet (#3032)
fix typo (#3108)
Port Scala Streamlet integration tests to Java (#3105)
[Java Streamlet API] Extend validations (#3095)
Fix build error on Mac. (#3102)
Add support for building Heron release in a Ubuntu 18.04 docker container (#3097)
Nwang/refactor custom operator interface v2 (#3100)
Fix number of tasks in Streamlet RemapCustomGrouping (#3098)
Update pidhandler.py (#3094)
enablebool (#3056)
Add interfaces IStreamletOperator, IStreamletBasicOperator and IStrea… (#3051)
Update S3 uploader documentations (#3053)
add back stmgr metrics (#3052)
Nwang/refactor grouping (#3040)