1.x: concatMap full rewrite + delayError + performance

[akarnokd]

This is a full rewrite of the concat/concatMap operator with the following properties:

• using OnSubscribe to save on the allocation of lift
• concatDelayError and concatMapDelayError to delay errors to the very end
• new trampolining logic that also subscribes to the next source even if requested is zero
• using concatMap or concatMapDelayError on a scalar source now bypasses the operator completely and uses ScalarSynchronousObservable.scalarFlatMap as well.
• the operator now detects just and empty sources and bypasses subscribing accordingly
• added ExceptionsUtils to work with composite exceptions and terminal state; useful for other similar delaying operators later on.
• concat is now implemented as concatMap(v -> v) which saves on the allocation of an entire map operator
• OperatorWindowWithSizeTest.testBackpressureOuterInexact had to be ignored because window doesn't honor backpressure properly and it is fixed in 1.x: fix counted buffer and window backpressure #3678; once it has been resolved, the test can be reenabled
• Added varargs constructor to CompositeException for convenience; avoids creating a list when the number of initial exceptions are known upfront.

Performance comparison to baseline (i7 4770K, Windows 7 x64, Java 8u72):

[akarnokd]

This is the benchmark comparison against Rsc:

[stevegury]

LGTM 👍