…ator

The original `async_generator.h` is quite heavy to include,
although many users need only a tiny fraction of what is
inside the header.

Provide forward declarations header (`async_generator_fwd.h`)
for basic `AsyncGenerator` and other generator classes,
so that users do not need to include the entire
`async_generator.h`, for example, if they only need a
`AsyncGenerator` typedef.

Signed-off-by: Pavel Solodovnikov <pavel.al.solodovnikov@gmail.com>

…ting fragments in Dataset

Add `GetFragmentsAsync()` and `GetFragmentsAsyncImpl()`
functions to the generic `Dataset` interface, which
allows to produce fragments in a streamed fashion.

This is one of the prerequisites for making
`FileSystemDataset` to support lazy fragment
processing, which, in turn, can be used to start
scan operations without waiting for the entire
dataset to be discovered.

To aid the transition process of moving to async
implementation in `Dataset`/`AsyncScanner` code,
a default implementation for `GetFragmentsAsyncImpl()`
is provided (iterating over `GetFragmentsImpl()`
via a `BackgroundGenerator` and transferring results
back to a given executor).

Tests: unit(release)

Signed-off-by: Pavel Solodovnikov <pavel.al.solodovnikov@gmail.com>

…etFragmentsAsync`

Provide two basic tests for `Dataset::GetFragments` and
`Dataset::GetFragmentsAsync` interfaces, utilizing
`InMemoryDataset` for testing purposes.

There was a helper function `AssertDatasetFragmentsEqual()`
for testing `GetFragments()` method, but it was unused until now,
meaning that the `Dataset` was essentially missing a part of
test coverage for `GetFragments()`.

This is fixed now by using this helper function in a
simple test case `TestInMemoryDataset::GetFragmentsSync`.

Analogous helper `AssertDatasetAsyncFragmentsEqual` is
introduced to iterate the dataset via `Dataset::GetFragmentsAsync()`
and is used in the `TestInMemoryDataset::GetFragmentsAsync` test-case.

Also, I have encountered a bug in
`DatasetFixtureMixin::AssertFragmentEquals`, which caused the
`GetFragmentsSync` test-case to fail:

The underlying fragment scanner always assumed to completely
drain the provided batch generator, but this is not the case with
`GetFragmentsSync` and `GetFragmentsAsync` test-cases, where each
fragment in the dataset is composed of a single batch from the source
batch generator.
Hence, the `AssertFragmentEquals()` should be called
as much times as there are fragments in the dataset, each time
advancing the batch generator position by a single batch.
But, the `AssertFragmentEquals()` would immediately fail after the
first iteration, because batch generator is not exhausted yet.

The bug is also fixed in this patch.

Tests: unit(release)

Signed-off-by: Pavel Solodovnikov <pavel.al.solodovnikov@gmail.com>