fix #29 Remove acquire() mono from pendings when cancelled
This is done by switching to a ConcurrentLinkedDeque for LIFO pools and
having Borrower#cancel remove self from the pending queue/stack.

fix #22 Add InstrumentedPool, with gauge-compatible metrics API
The idea in this PR is to expose internal state of the pool through a PoolMetrics API (similar to what is done with PooledRef, a view of the Pool).

It also exposes 2 configuration elements (max size and max pending size).

The former UNBOUNDED allocation strategy must now be instantiated independently
in order to keep track of current allocation count for the benefit of these gauges.

fix #1558 Add ToStringConverter and Extractor to StepVerifierOptions
This commit allows users to redefine `String` representation of various
objects that can be included in StepVerifier-produced messages:

 - added 2 new interfaces with implementations in ValueFormatters util
 - StepVerifierOptions can be set up with a single class-matching
 ToStringConverter, along with 0-n Extractors for container values
 - ErrorFormatter is now MessageFormatter
 - it has a more generic format(msg, arg...) method, which is preferred
 to String.format in the DefaultStepVerifier

In options, `getExtractors()` produces a copy of the user-defined
extractors to which it adds defaults at the end. Since only the first
matching extractor applies, this ensures that eg. adding an extractor
for `Integer[]` won't be superseded by the default Object[] extractor.

Add an API-preserving hide() method to ConnectableFlux (#1577)

fix #1652 Unwrap CompletionException in Mono#fromFuture
The CompletionException wraps the original Throwable, but only when it
is thrown from within user-provided functions transforming the
CompletionStage.

However, as Subscriber#onError gladly accepts a Throwable, it makes
sense to unwrap the CompletionException for the benefit of the
downstream Subscriber.

fix #1647 Null out lastKey to avoid retaining in distinctUntilChanged
This includes cancel, complete and error.

fix #29 Remove acquire() mono from pendings when cancelled
This is done by switching to a ConcurrentLinkedDeque for LIFO pools and
having Borrower#cancel remove self from the pending queue/stack.

fix #22 Add InstrumentedPool, with gauge-compatible metrics API
The idea in this PR is to expose internal state of the pool through a PoolMetrics API (similar to what is done with PooledRef, a view of the Pool).

It also exposes 2 configuration elements (max size and max pending size).

The former UNBOUNDED allocation strategy must now be instantiated independently
in order to keep track of current allocation count for the benefit of these gauges.