Import::Caching sets TTL greater than existing

What does this MR do and why?

This MR changes the Gitlab::Cache::Import::Caching.read, .increment, and .increment_by methods to now only set an expiry time for a key if it is greater than the existing TTL.

This fixes a bug where .read would set the TTL to a much lower value in some situations #427097 (comment 1615773392)

How to set up and validate locally

Gitlab::Cache::Import::Caching.write('134940:write', 'bar')
Gitlab::Cache::Import::Caching.read('134940:write', timeout: 1)
# Wait 2 seconds...
Gitlab::Cache::Import::Caching.read('134940:write', timeout: 1) # => Will return `"bar"` on this branch, but `nil` on master

Gitlab::Cache::Import::Caching.increment_by('134940:increment_by', 1)
Gitlab::Cache::Import::Caching.increment_by('134940:increment_by', 1, timeout: 1)
# Wait 2 seconds...
Gitlab::Cache::Import::Caching.read('134940:increment_by') # => Will return `"2"` on this branch, but `nil` on master

Gitlab::Cache::Import::Caching.increment('134940:increment')
Gitlab::Cache::Import::Caching.increment('134940:increment', timeout: 1)
# Wait 2 seconds...
Gitlab::Cache::Import::Caching.increment('134940:increment') # => Will return `3` on this branch, but `1` on master

MR acceptance checklist

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