feat(sdk): Mapping concurrency

[teogeb]

Improved concurrency for Mapping class. Now concurrent equal get() calls share the same promise, i.e. they evaluate the valueFactory only once.

Example

const valueFactory = async () => ...
const mapping = new Mapping(valueFactory)
await Promise.all([
    mapping.get('foo'),
    mapping.get('bar'),
    mapping.get('bar'),
    mapping.get('foo'),
    mapping.get('foo')
])

Triggers two calls to valueFactory (one for foo and another for bar). Before this PR it triggered 5 calls.

Small refactoring

• Added readonly field annotations
• Updated deprecated jest method call