Fix lint warnings

[vr009]

code health: fix all places highlighted by linter

Changed the warning's suppression in check.yaml. The suppression of the rule
errcheck may be removed after adding errors check in all methods
with calling encodeXxx inside. See details below.

Suppressed the highlighting lacks of error's check in all methods,
having encodeXxx inside. For now those methods are not able to
return any error due to internal implementation of writer
interface (see smallbuf.go). For future, if the implementation of the writer
will be changed, and there will be a need to check errors, we must think
about how to say to compiler that the error check is 'unlikely' for keeping performance
(If there will be any affect on it).

Fixed the use of time package API in all places with calls of
time.Now().Sub(). Now time package propose the explicit time.Until() and time.Since().

Replaced all calls of Errorf() with Fatalf() in tests, where it is
needed. That change prevents nil dereferences below in the code and
stops the test execution, where it is expected in tests.

Suppressed the highlighting of all unused constants and functions with //nolint comment.

Fixed the calling of Fatalf() from non-testing goroutine in queue tests.
It is not a valid way to stop test from another goroutine.

Fixed the gofmt-highlighted places in test_helpers/pool_helper.go and
connection_pool/const.go.

Added instructions to CONTRIBUTING.md how to run CI-linter locally.

Closes #142
Closes #150

[ligurio]

Replaced some calls of Errorf() with Fatalf() in tests. That change
prevents nil dereferences below in the code.

It looks strange that goling-ci warns for a few t.Errorf because we have about several dozens t.Errorf() in tests (and we have plans to fix it in #150):

sergeyb@pony:~/sources/go/src/github.com/tarantool/go-tarantool$ find . -name "*_test.go" | xargs grep "t.Errorf" | wc -l
316

[ligurio]

in commit message:

Relates to #142

It should be "Closes #142"

[ligurio]

In general, I'm ok with changes, but please fix commit message and godoc comments before merge.

[ligurio]

In general, I'm ok with changes, but please fix commit message and godoc comments before merge.

[DifferentialOrange]

code health: fix all places highlighted by linter

It is good that there are many people working on Go connector right now, but it also means their work should be synchronized. For example, I think that big nice PR #148 will be merged before that one, and there could be some new lint warnings. So it seems that you need to sync with other contributors so

• this PR fixes all warnings in current master
• all other PRs are rebased to new master and contain no new warnings

[DifferentialOrange]

I have left a several comments. As I remember from talking with Sergey, there really isn't many questionable places left, so I think the review won't spend too many time.

[DifferentialOrange]

Please, rebase to master. It is possible that master discovery commit contains some new warnings

[Totktonada]

I'm ok after resolving my comments.