MarketCap calculation: check that ETS tables exist before inserting new data or lookup from the table

[vbaranov]

Motivation

The appearance of error like this

Request: GET /
** (exit) an exception was raised:
    ** (ArgumentError) argument error
        (stdlib 3.12.1) :ets.insert(:bridges_market_cap, {"current_market_cap_from_omni_bridge", #Decimal<0E-18>})
        (explorer 0.0.1) lib/explorer/counters/bridge.ex:152: Explorer.Counters.Bridge.update_total_omni_bridge_market_cap_cache/0
        (explorer 0.0.1) lib/explorer/chain/supply/token_bridge.ex:60: Explorer.Chain.Supply.TokenBridge.market_cap/1
        (block_scout_web 0.0.1) lib/block_scout_web/templates/chain/show.html.eex:66: BlockScoutWeb.ChainView."show.html"/1
        (phoenix 1.5.4) lib/phoenix/view.ex:310: Phoenix.View.render_within/3
        (phoenix 1.5.4) lib/phoenix/view.ex:472: Phoenix.View.render_to_iodata/3
        (phoenix 1.5.4) lib/phoenix/controller.ex:776: Phoenix.Controller.render_and_send/4
        (block_scout_web 0.0.1) lib/block_scout_web/controllers/chain_controller.ex:1: BlockScoutWeb.ChainController.action/2
2020-10-07T13:47:23.725 [error] Task Explorer.ExchangeRates started from #PID<0.23441.0> terminating

Changelog

Before inserting or lookup in ETS table, check that ETS table exists. Sometimes due to race conditions initialization of the process which creates those tables happens later than searching from/insertion to those tables requested from other processes.

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 dc8f49a48b46f5391c5a999d050bd7912e5a2a37-PR-3335

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 8 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+6.8%) to 74.369%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/channels/address_channel.ex	2	36.96%
lib/block_scout_web/checksum_address.ex	6	55.56%

Totals
Change from base Build d2ec9d28e5c68453ab422ebd5f2373affd5d91ca:	6.8%
Covered Lines:	1973
Relevant Lines:	2653

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💛 - Coveralls