feat(core): implement auto-flush mode

[B4nan]

Checks state before querying entities and flush automatically if we have queued changes to entities that are being queried.

The flushing strategy is given by the flushMode of the current running EntityManager.

• FlushMode.COMMIT - The EntityManager tries to delay the flush until the current Transaction is committed, although it might flush prematurely too.
• FlushMode.AUTO - This is the default mode, and it flushes the EntityManager only if necessary.
• FlushMode.ALWAYS - Flushes the EntityManager before every query.

FlushMode.AUTO will try to detect changes on the entity we are querying, and flush
if there is an overlap:

// querying for author will trigger auto-flush if we have new author persisted
const a1 = new Author(...);
orm.em.persist(a1);
const r1 = await orm.em.find(Author, {});

// querying author won't trigger auto-flush if we have new book, but no changes on author
const b4 = new Book(...);
orm.em.persist(b4);
const r2 = await orm.em.find(Author, {});

// but querying for book will trigger auto-flush
const r3 = await orm.em.find(Book, {});

We can set the flush mode on different places:

• in the ORM config via Options.flushMode
• for given EntityManager instance (and its forks) via em.setFlushMode()
• for given EntityManager fork via em.fork({ flushMode })
• for given QueryBuilder instance via qb.setFlushMode()
• for given transaction scope via em.transactional(..., { flushMode })

Based on https://docs.jboss.org/hibernate/orm/5.1/userguide/html_single/chapters/flushing/Flushing.html

Closes #2359

[codecov-commenter]

Codecov Report

Merging #2491 (f609d48) into master (a0827f5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2491   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          190       190           
  Lines        10874     10958   +84     
  Branches      2489      2502   +13     
=========================================
+ Hits         10874     10958   +84     

Impacted Files	Coverage Δ
packages/core/src/drivers/IDatabaseDriver.ts	100.00% <ø> (ø)
packages/core/src/enums.ts	100.00% <ø> (ø)
packages/core/src/EntityManager.ts	100.00% <100.00%> (ø)
packages/core/src/entity/BaseEntity.ts	100.00% <100.00%> (ø)
packages/core/src/entity/EntityFactory.ts	100.00% <100.00%> (ø)
packages/core/src/entity/EntityHelper.ts	100.00% <100.00%> (ø)
packages/core/src/entity/WrappedEntity.ts	100.00% <100.00%> (ø)
packages/core/src/typings.ts	100.00% <100.00%> (ø)
packages/core/src/unit-of-work/UnitOfWork.ts	100.00% <100.00%> (ø)
packages/core/src/utils/Configuration.ts	100.00% <100.00%> (ø)
... and 5 more

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