Selective Flushing on Writes

[eytan-avisror]

Currently, when a write is made, it will flush all service caches.
For example, a call to elasticloadbalancing.DescribeTargetHealth will cache, and a subsequent call to elasticloadbalancing.DeregisterTarget (an non-cacheable write) would flush all caches under elasticloadbalancing.

This behavior might be inappropriate if the write is irrelevant to other reads.
In the above case for example, I do not want DeregisterTargets to flush all my caches for elasticloadbalancing.

This is further relevant when considering that elb and elbv2 are both using service name elasticloadbalancing.

This PR adds a map of mutatingCaches which can define which write operations are mutating and we should flush when they are called.

By default, all writes are considered mutating (to have same behavior as now), however one could choose to exclude a specific operation from this behavior by using SetCacheMutating(serviceName, operationName, false) => this will mark the specific operation as non-mutating.
In this case we will only flush the operation cache i.e. discard that write cache instead of flushing all service caches

This can really improve cache hit/miss in some scenarios.

[coveralls]

Pull Request Test Coverage Report for Build 33

• 29 of 31 (93.55%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.8%) to 72.596%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cache/config.go	29	31	93.55%

Totals
Change from base Build 31:	2.8%
Covered Lines:	151
Relevant Lines:	208

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💛 - Coveralls