Migrate FsShardsCheck healthcheck to work with gitaly

File: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/health_checks/fs_shards_check.rb

FsShardsCheck still uses stat against the local filesystem, which prevents us from using the liveness and readiness endpoints in unicorn container probes in the cloud native project.

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Stages:

• RPC Design: gitaly-proto!197 (merged)

• ~"Client Implementation": https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20022 and https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6205

• Server Implementation: !767 (merged)

• ~"Acceptance Testing": Create Issue

RPC Endpoints:

• Endpoint::Name

Known Client Routes:

• Known client endpoints

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