[MEX-527] Memory Store Apollo plugin

[mad2sm0key]

Reasoning

• the API sufferes from performance degradation in high concurrent requests scenarios. This happens due to the high number of fields with resolver functions and the highly nested nature of certain models.
• the method propsed below, is meant to reduce load with the following strategy :

1. maintain certain complex models in a singleton (memory store)
2. hook into the graphQL lifecycle (bypass the regular resolve route) and return the data directly from the memory store

• this PR tackles point 2

Proposed Changes

• add singleton that will hold all pairs, all esdt tokens and the mapping pair<>esdt_tokens in memory
• add interface (actually abstract class) that describes the behaviour of a service capable of returning response from the memory store based on a graphQL query
• add a memory store factory service that abstracts away the underlying memory store services
• add Apollo server plugin that attempts to resolve queries (partially or completely) from the memory store using the memory store factory
• append -store_partial or -store_full to the metrics key of queries resolved from the memory store

How to test

• N/A