Lossless Sampling

[christos-h]

Required Functionality

Currently the XExportStrategy and Sampler::sample functions work with vectors of JSON values.

This is handy, but it is loses information.

In fact for any data sink which has types which are a superset of the JSON data model (both Postgres and Mongo) you will be losing information as most types get serialized to a string (for example timestamps).

This can be a problem, as at insertion time you don't know what type to use on the client library.

Proposed Solution

1. It feels like export strategy is doing too much + too tightly coupled to the sampler
2. The sampler returns JSON values, which is not ideal. We want the sampler to return a vector of Value types in core::graph.
3. The type mapping has to be redone. We currently have JSON -> PG types & JSON -> Mongo. This needs to be re-implemented for core::graph::Value -> X.

[llogiq]

While we're at it, we should probably look into #28 regarding escaping (at least for Postgres).