investigate and fix staticcheck issues

[dmitris]

The output of $ golangci-lint run --disable-all -E staticcheck ./...:
https://gist.github.com/dmitris/9e0dcaeba2c55ada6071dc844a7f7df0

It would be great to go through those issues, decide which ones should be addressed and fix them.

[dmitris]

current output (commit 4dc4005):

$ golangci-lint run --disable-all -E staticcheck ./... |& tee /tmp/out
stored_requests/events/http/http.go:83:15: SA1015: using time.Tick leaks the underlying ticker, consider using it only in endless functions, tests and the main package, and use time.NewTicker here (staticcheck)
	go e.refresh(time.Tick(refreshRate))
	             ^
adapters/yandex/yandex.go:159:40: SA1019: yandexExt.ImpID is deprecated: in favor of `PlacementID` (staticcheck)
		placementID.ImpID = strconv.Itoa(int(yandexExt.ImpID))
		                                     ^
adapters/yandex/yandex.go:160:41: SA1019: yandexExt.PageID is deprecated: in favor of `PlacementID` (staticcheck)
		placementID.PageID = strconv.Itoa(int(yandexExt.PageID))
		                                      ^
server/ssl/ssl_test.go:15:14: SA1019: certPool.Subjects has been deprecated since Go 1.18: if s was returned by [SystemCertPool], Subjects will not include the system roots. (staticcheck)
	subjects := certPool.Subjects()
	            ^
server/ssl/ssl_test.go:25:13: SA1019: certPool.Subjects has been deprecated since Go 1.18: if s was returned by [SystemCertPool], Subjects will not include the system roots. (staticcheck)
	subjects = certPool.Subjects()
	           ^
server/ssl/ssl_test.go:40:14: SA1019: certPool.Subjects has been deprecated since Go 1.18: if s was returned by [SystemCertPool], Subjects will not include the system roots. (staticcheck)
	subjects := certPool.Subjects()
	            ^
adapters/rubicon/rubicon.go:387:4: SA4006: this value of `err` is never used (staticcheck)
			videoCopy.Ext, err = json.Marshal(&videoExt)
			^
adapters/rubicon/rubicon.go:450:4: SA4006: this value of `err` is never used (staticcheck)
			siteCopy.Publisher.Ext, err = json.Marshal(&pubExt)
			^
analytics/agma/sender_test.go:125:2: SA4006: this value of `err` is never used (staticcheck)
	sender, err := createHttpSender(client, config.AgmaAnalyticsHttpEndpoint{
	^
adapters/adgeneration/adgeneration_test.go:248:5: SA5011(related information): this check suggests that the pointer can be nil (staticcheck)
	if bidderResponse == nil {
	   ^
adapters/adgeneration/adgeneration_test.go:264:40: SA5011: possible nil pointer dereference (staticcheck)
	assert.Equal(t, 1, len(bidderResponse.Bids))
	                                      ^
adapters/adgeneration/adgeneration_test.go:265:45: SA5011: possible nil pointer dereference (staticcheck)
	assert.Equal(t, expectedID, bidderResponse.Bids[0].Bid.ID)
	                                           ^
adapters/adgeneration/adgeneration_test.go:266:48: SA5011: possible nil pointer dereference (staticcheck)
	assert.Equal(t, expectedImpID, bidderResponse.Bids[0].Bid.ImpID)
	                                              ^
exchange/bidder_test.go:1068:20: SA5011(related information): this check suggests that the pointer can be nil (staticcheck)
		assert.Falsef(t, seatBid == nil && tc.expectedBidsCount != 0, tc.description)
		                 ^