Workhorse: remove the global Redis client

What does this MR do and why?

While running tests locally for the first time, I got a unit test panic:

=== RUN   TestKeyChangesInstantReturn
=== RUN   TestKeyChangesInstantReturn/sees_change_with_key_existing_and_changed
--- FAIL: TestKeyChangesInstantReturn (0.00s)
    --- FAIL: TestKeyChangesInstantReturn/sees_change_with_key_existing_and_changed (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x8 pc=0x1054b0cc0]

goroutine 13 [running]:
testing.tRunner.func1.2({0x105761a80, 0x106029ed0})
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1545 +0x1c8
testing.tRunner.func1()
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1548 +0x360
panic({0x105761a80?, 0x106029ed0?})
	/opt/homebrew/opt/go/libexec/src/runtime/panic.go:914 +0x218
gitlab.com/gitlab-org/gitlab/workhorse/internal/redis.TestKeyChangesInstantReturn.func1(0x140000be701?)
	/Users/mike/src/gitlab/workhorse/internal/redis/keywatcher_test.go:118 +0x50
testing.tRunner(0x140005f3040, 0x140006079a0)
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1595 +0xe8
created by testing.(*T).Run in goroutine 12
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1648 +0x33c

So, I decided to fix it by checking errors properly in initRdb() and cleanly reporting them. Got this, which is much better than a panic:

=== RUN   TestKeyChangesInstantReturn
    keywatcher_test.go:23: 
        	Error Trace:	/Users/mike/src/gitlab/workhorse/internal/redis/keywatcher_test.go:23
        	            				/Users/mike/src/gitlab/workhorse/internal/redis/keywatcher_test.go:79
        	Error:      	Received unexpected error:
        	            	open ../../config.toml: no such file or directory
        	Test:       	TestKeyChangesInstantReturn
--- FAIL: TestKeyChangesInstantReturn (0.00s)

Then I decided to have a more holistic look at the code as that package-level field looks fishy. I think it's much nicer without it.

Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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