Improving perf when getting limits in storage

[didierofrivia]

This is part of #183

Even tho it's an apparent simple change, the performance improvement is tangible:

Flame graph to compare the overhead simplification

To the left, before the change, then this PR change. It also shows the search for Limit as core::hash

Criterion performance results for In Memory storage, check_rate_limited_and_update function

1 namespaces with 1 limits each with 1 conditions and 1 variable

10 namespaces with 10 limits each with 10 conditions and 10 variables

10 namespaces with 50 limits each with 10 conditions and 10 variables

Notes:

• From about 6 runs, this one was the least performant
• The most performant went up to 28 estimate for 10 ns, 50 lim each with 10 conditions and 10 variables

[alexsnaps]