Add `--rpc.gaspricemultiplier` option

[palango]

This add command line option to adjust the multiplier in the gas price suggestion RPC endpoint.

Example:

geth --rpc.gaspricemultiplier 1.456

Resolves #2136

Further improvements are tracked in #2153

[github-actions[bot]]

[codecov[bot]]

Codecov Report

Patch coverage: 35.71% and project coverage change: +0.03 :tada:

Comparison is base (88635e4) 55.26% compared to head (b58489e) 55.30%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2154 +/- ## ========================================== + Coverage 55.26% 55.30% +0.03% ========================================== Files 675 675 Lines 113729 113739 +10 ========================================== + Hits 62853 62902 +49 + Misses 47027 46996 -31 + Partials 3849 3841 -8 ``` | [Impacted Files](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org) | Coverage Δ | | |---|---|---| | [cmd/geth/main.go](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org#diff-Y21kL2dldGgvbWFpbi5nbw==) | `21.39% <ø> (ø)` | | | [cmd/geth/usage.go](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org#diff-Y21kL2dldGgvdXNhZ2UuZ28=) | `10.90% <ø> (ø)` | | | [cmd/utils/flags.go](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org#diff-Y21kL3V0aWxzL2ZsYWdzLmdv) | `2.54% <0.00%> (-0.02%)` | :arrow_down: | | [eth/api\_backend.go](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org#diff-ZXRoL2FwaV9iYWNrZW5kLmdv) | `0.00% <0.00%> (ø)` | | | [les/api\_backend.go](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org#diff-bGVzL2FwaV9iYWNrZW5kLmdv) | `0.00% <0.00%> (ø)` | | | [contracts/gasprice\_minimum/gasprice\_minimum.go](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org#diff-Y29udHJhY3RzL2dhc3ByaWNlX21pbmltdW0vZ2FzcHJpY2VfbWluaW11bS5nbw==) | `49.39% <100.00%> (+1.89%)` | :arrow_up: | ... and [31 files with indirect coverage changes](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2154/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org)

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[github-actions[bot]]

Coverage from tests in ./e2e_test/... for ./consensus/istanbul/... at commit be7fd44ad5198611ac896c9215ecd5161f018869

coverage: 48.4% of statements across all listed packages

coverage:  60.0% of statements in consensus/istanbul
coverage:  40.5% of statements in consensus/istanbul/announce
coverage:  54.5% of statements in consensus/istanbul/backend
coverage:   0.0% of statements in consensus/istanbul/backend/backendtest
coverage:  24.3% of statements in consensus/istanbul/backend/internal/replica
coverage:  60.2% of statements in consensus/istanbul/core
coverage:  45.0% of statements in consensus/istanbul/db
coverage:   0.0% of statements in consensus/istanbul/proxy
coverage:  64.4% of statements in consensus/istanbul/uptime
coverage:  51.8% of statements in consensus/istanbul/validator
coverage:  79.2% of statements in consensus/istanbul/validator/random