minor style fixes

[quasilyte]

• For if/else chain, switch statement is usually considered more idiomatic
• If http package already imported, one may use its constants for method type
• Params with same type can be combined

elector.go:131:2: elseif: should rewrite if-else to switch statement
elector.go:122:36: stdExpr: can replace "GET" with net/http.MethodGet
elector.go:179:35: stdExpr: can replace "PUT" with net/http.MethodPut
elector.go:87:1: paramTypeCombine: func(method string, url string, reqBody []byte) (resp *http.Response, resBody []byte, err error) could be replaced with func(method, url string, reqBody []byte) (resp *http.Response, resBody []byte, err error)
elector.go:246:1: paramTypeCombine: func(selfId string, consulUrl string, leaderHoldTime time.Duration) (inst *Instance, err error) could be replaced with func(selfId, consulUrl string, leaderHoldTime time.Duration) (inst *Instance, err error)

[afiskon]

I see no reason why this PR should not be merged :) Thank you!