Simplify CryptoSSLRequestManager using unified futures API
Simplify CryptoSSLRequestManager using unified futures API

Summary:
This diff unifies the nearly-replicated call paths for ECDSA and RSA
sign request/response/fallback/etc. into one generic futures-based sign API.
Once this is tested, we should be able to remove ~500 lines of code from
CryptoSSLRequestManager

Depends on D3094660

Reviewed By: siyengar

Differential Revision: D3116535

fb-gh-sync-id: 11c50d81dab39e5426679b5a172c1ece2039fc10
fbshipit-source-id: 11c50d81dab39e5426679b5a172c1ece2039fc10

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