Fix find block timestamp query

[vbaranov]

Motivation

An error like this:

2020-10-20T12:47:04.925 [error] #PID<0.629.6> running BlockScoutWeb.Endpoint (connection #PID<0.628.6>, stream id 1) terminated
Request: GET /address/0x0B02534634092077Fbf73431867DB83A7B008084/coin-balances?type=JSON
** (exit) an exception was raised:
    ** (Ecto.MultipleResultsError) expected at most one result but got 2 in query:

from b0 in Explorer.Chain.Block,
  where: b0.number == ^12609315,
  select: b0.timestamp

        (ecto 3.3.4) lib/ecto/repo/queryable.ex:115: Ecto.Repo.Queryable.one/3
        (explorer 0.0.1) lib/explorer/chain.ex:4390: Explorer.Chain.address_to_coin_balances/2
        (block_scout_web 0.0.1) lib/block_scout_web/controllers/address_coin_balance_controller.ex:23: BlockScoutWeb.AddressCoinBalanceController.index/2
        (block_scout_web 0.0.1) lib/block_scout_web/controllers/address_coin_balance_controller.ex:1: BlockScoutWeb.AddressCoinBalanceController.action/2
        (block_scout_web 0.0.1) lib/block_scout_web/controllers/address_coin_balance_controller.ex:1: BlockScoutWeb.AddressCoinBalanceController.phoenix_controller_pipeline/2
        (phoenix 1.5.4) lib/phoenix/router.ex:352: Phoenix.Router.__call__/2
        (phoenix 1.5.4) lib/phoenix/router/route.ex:41: Phoenix.Router.Route.call/2
        (phoenix 1.5.4) lib/phoenix/router.ex:352: Phoenix.Router.__call__/2

Changelog

Limit query results to one

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 67b26f376cf4a55a97319c5124fb4f8db34ec544-PR-3374

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-1.0%) to 67.591%

---

Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/channels/address_channel.ex	2	36.96%
lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	2	71.43%

Totals
Change from base Build 59085048dcb1851aa4c1b7394074cc2ce030e7ce:	-1.0%
Covered Lines:	2048
Relevant Lines:	3030

---

💛 - Coveralls