Allow to set logger to nil

[ivanvc]

The implementation doesn't look pretty. But now, when the logger is not defined (rather than using the discardLogger, skip logging altogether). By doing this, the main branch restores the benchmark results from release 1.3.

Related to #720.

[ivanvc]

/cc @ahrtr @fuweid @tjungblu @Elbehery

[ahrtr]

There are some places that you do not check if lg != nil, for example

• (*DB) grow(...): line 1234/1256/1261 in db.go
• (*DB) Sync(): line 1132 in db.go
• Open: line 276 in db.go
• ......

It's really error prone. Some code branches are not covered by test, so the issue may not be discovered until running in production environment.

Probably the second solution mentioned in #720 (comment) is safer.

[ivanvc]

There are some places that you do not check if lg != nil, for example

* (*DB) grow(...): line 1234/1256/1261 in db.go

* (*DB) Sync(): line 1132 in db.go

* Open: line 276 in db.go

* ......

It's really error prone. Some code branches are not covered by test, so the issue may not be discovered until running in production environment.

Probably the second solution mentioned in #720 (comment) is safer.

You're right; I realize I missed it in some places, e.g., db.Sync(). It definitely is more error-prone. I can review all log lines again if we want to take this approach. If we decide not to, then I don't think it's worth addressing all log lines.

[ivanvc]

Closing this PR. I'll open a new PR following the suggested approach.