Sync active memtable and value log on Db.Sync (#1847)

[SarthakMakhija]

Fixes #1847

Currently, DB.Sync() only syncs the value log but not the WAL of the active memtable, however recovery happens from the WAL of the active memtable as a part of the DB.Open() method.

This change attempts to sync both the logs, however there can be an issue in syncing one of the logs. This change lists all of the possible cases that can arise during the sync operations. Some of these issues will be taken separately. For example,
the issue #1954 can arise and shall be handled separately.

This change adds a few tests to ensure that the Sync behavior is not broken. This change also adds a learning test (db2_test.go -> TestAssertValueLogIsNotWrittenToOnStartup) to ensure that value log is only read from (and not written to) during startup. This learning shall be used in the next issue (the one described above (*))

[CLAassistant]

All committers have signed the CLA.

[mangalaman93]

I just realized that we may need to consider acquiring some lock while accessing the memtable and vlog. Please look into it. Also, address the comments from the CI. CI also is not happy.

[SarthakMakhija]

I just realized that we may need to consider acquiring some lock while accessing the memtable and vlog. Please look into it. Also, address the comments from the CI. CI also is not happy.

Linting part is ok, I will reduce the line length in the comment.
Pipeline is failing because of the coverage

FAIL
coverage: 64.8% of statements

I am not sure if I can cover this part.

[mangalaman93]

I am not sure if I can cover this part.

Don't worry about it for now.

[mangalaman93]

I am happy with the change now. I will let @joshua-goldstein approve it for merging after review. Thanks for the PR Sarthak.

[SarthakMakhija]

I am happy with the change now. I will let @joshua-goldstein approve it for merging after review. Thanks for the PR Sarthak.

Welcome Aman :). Thank you for all the help.