Smaller cleanups or perf changes

[koivunej]

This PR will:

• change all crates to use the interledger-packet::hex::HexString introduced in #690 instead of hex::encode which produces a String
  • hex will still be used in ilp-node for configuration and test support
• change all crates to use hex_literal::hex! or &[u8] for static test content instead of Lazy<Vec<u8>>
• use &'static str for the btp subprotocol names which are listed in the spec (assumed most often used)
• instead of cloning the whole connections map in clone the single needed sender

This work came out of #690 and follow-up work.

[koivunej]

Merging this as this is very much uncontroversial, and most likely non-breaking (interledger-btp::packet is not public).