fix GEARPUMP-32, introduce source watermark

[manuzhang]

This PR introduces source watermark such that

1. messages with timestamps earlier than watermark could be filtered out at source.
2. minclock of source processor will be bounded by watemark

[manuzhang]

The PR is not mature and opened early to gather thoughts from the community.

[codecov-io]

Current coverage is 65.96%

Merging #33 into master will increase coverage by 2.45%

1. 5 files (not in diff) in ...rpump/streaming/task were modified. more
2. 2 files (not in diff) in ...streaming/state/impl were modified. more
3. 2 files (not in diff) in .../streaming/state/api were modified. more
4. 2 files (not in diff) in ...mp/streaming/metrics were modified. more
5. 2 files (not in diff) in ...p/streaming/executor were modified. more
   • Misses -1
   • Hits +1
6. 2 files (not in diff) in ...p/streaming/dsl/plan were modified. more
7. 1 files (not in diff) in ...arpump/streaming/dsl were modified. more
8. 3 files (not in diff) in .../streaming/appmaster were modified. more
9. 1 files (not in diff) in ...e/gearpump/streaming were modified. more
10. 2 files (not in diff) in .../lib/source/consumer were modified. more

@@             master        #33   diff @@
==========================================
  Files           178        178          
  Lines          5922       5929     +7   
  Methods        5644       5631    -13   
  Messages          0          0          
  Branches        278        298    +20   
==========================================
+ Hits           3761       3911   +150   
+ Misses         2161       2018   -143   
  Partials          0          0          

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