Make Storage API and implementation more friendly to Spine's storage-specific libraries

[armiol]

This PR addresses some inconsistencies in both Storage API and its base/abstract implementation, so that descendant port libraries, such as Spine RDBMS could migrate their features to Spine 2.x with less hassle.

Major breaking change (breaking for SPI users, that is) is merging EntityRecordSpec and MessageRecordSpec into a single RecordSpec type. It has been done for several reasons.

• Allow to assemble the columns for EntityRecords consistently on top of EntityRecord itself. Previously, the column values in EntityRecordWithColumns — which were picked from Entity state directly — could differ from those values actually stored in EntityRecord.
• Provide an ability to re-assemble RecordWithColumns instances purely with the relevant Message instance along with its spec. Previously, Entity-related RecordWithColumns could only be assembled with an Entity instance.
• Simplify the code in Storage API, eliminating one of type inheritance branches.

Other changes include:

• Storage column definitions made public.
• Transformation of generic entity queries into Storage-internal RecordQuery instances now preserves the type of the column, by which values query aims to filter the dataset.
• Behaviour of AggregateStorage.historyBackward() has been made eager instead of lazy, as previously. This is needed to ensure the access to the history under the circumstances of maintaining the connection to the database.
• More documentation was added to the tenancy modes recommended for certain scenarios, such as configuring system storages (e.g. InboxStorage).
• FieldMaskApplier has been made @Internal-ly public, as its functionality is required in descendant libraries to prevent any code duplication.

[armiol]

@alexander-yevsyukov PTAL.

[codecov[bot]]

Codecov Report

Merging #1533 (e4ca6ea) into master (349fc83) will decrease coverage by 0.08%. The diff coverage is 96.98%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1533 +/- ## ============================================ - Coverage 89.90% 89.83% -0.08% + Complexity 5057 5025 -32 ============================================ Files 648 645 -3 Lines 15806 15751 -55 Branches 925 920 -5 ============================================ - Hits 14211 14150 -61 - Misses 1269 1274 +5 - Partials 326 327 +1 ```

[armiol]

@alexander-yevsyukov it has been a long time since this PR was first approved. Many things have changed since then. PTAL again.

[armiol]

@alexander-yevsyukov while reviewing the ... .storage package, I have also addressed some minor documentation issues, killed some unused methods, and commented on warnings.

And considering @SPI-ness, I have added an explicit test for RecordWithColumns API.

[armiol]

@alexander-yevsyukov PTAL.