Remove TLS dependency on TCP

[turuslan]

• Remove dynamic_cast.
• Can use TLS over any libp2p stream.
• Can use in WASM (browsers don't allow TCP).

Open questions:

• std::error_code and boost::system::error_code conversion.
  • Inconsistent conversions may create memory leaks (e.g. s1 -> b1, b1 -> s1', s1' -> b1', ...).
    • Boost makes std::error_category wrapper for boost::system::error_category. It proxies name() and message() calls. But we can't inject original std::error_category into this mapping. So even if we maintain our own mapping, but some code casts error via boost, it may create new wrappers indefinitely.
  • Error comparison may be unavailable (e.g. error == EOF) for underlying stream errors.
    • If we will map to single boost error.
    • Or if conversion is inconsistent (e.g. s1 -> b1, b1 -> s1', s1 != s1').
  • May store last read and write errors in stream instance (like errno), as there can only be one active read/write.
• May need to check lifetimes during async read/write operation.