Add expression and expressionBatch window

[suhothayan]

Purpose

To hold values in the window only until the given expression is satisfied. The expression window expires the oldest events until the given expression is not satisfied, and the expressionBatch window expires all events as a batch when the given expression is not satisfied.
Here the expression should be passed as a string, as it is executed within the window.
Fixes #1596

Goals

Support event expiry based on an expression.

Approach

Expression and ExpressionBatch window evaluates the events from the oldest (first) to the newest (last) and expires them if they do not satisfy the condition.
E.g.

from StockEventWindow#window.expression(
          'sum(price) < 100 and eventTimestamp(last) - eventTimestamp(first) < 3000')
select symbol, sum(price) as price
insert into OutputStream ;

Retain only the events having their sum(price) < 100, and the last and first events are within 3-second difference in a sliding manner.

from StockEventWindow#window.expressionBatch('last.symbol==first.symbol')
select symbol, sum(price) as price
insert into OutputStream ;

Output events as a batch when a new symbol type arrives.

[codecov-io]

Codecov Report

Merging #1599 into master will increase coverage by 0.13%.
The diff coverage is 76.07%.

@@             Coverage Diff              @@
##             master    #1599      +/-   ##
============================================
+ Coverage     67.73%   67.86%   +0.13%     
  Complexity       45       45              
============================================
  Files           651      654       +3     
  Lines         27524    27982     +458     
  Branches       4340     4403      +63     
============================================
+ Hits          18643    18990     +347     
- Misses         7062     7162     +100     
- Partials       1819     1830      +11

Impacted Files	Coverage Δ	Complexity Δ
...re/src/main/java/io/siddhi/core/window/Window.java	71.92% <ø> (-0.25%)	0 <0> (ø)
...processor/stream/window/HopingWindowProcessor.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...ocessor/stream/window/GroupingWindowProcessor.java	96% <100%> (+1.26%)	0 <0> (ø)	⬇️
...ocessor/stream/window/BatchingWindowProcessor.java	100% <100%> (ø)	0 <0> (ø)	⬇️
...ddhi/core/util/parser/SingleInputStreamParser.java	87.82% <100%> (+0.21%)	0 <0> (ø)	⬇️
...a/io/siddhi/core/event/stream/MetaStreamEvent.java	94.36% <100%> (-4.17%)	0 <0> (ø)
.../io/siddhi/core/util/parser/AggregationParser.java	75.27% <100%> (ø)	0 <0> (ø)	⬇️
...cutor/function/EventTimestampFunctionExecutor.java	80% <100%> (+67.5%)	0 <0> (ø)	⬇️
...io/siddhi/core/aggregation/AggregationRuntime.java	53.11% <100%> (ø)	0 <0> (ø)	⬇️
...rocessor/stream/window/SlidingWindowProcessor.java	100% <100%> (ø)	0 <0> (ø)	⬇️
... and 18 more

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