Add tx_search endpoint for RPC client

[thanethomson]

Closes #698

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md

[greg-szabo]

Generally looks good.
I'm happy to see the correct use of our slightly messy serialization solution :) and I'm even happier to see rpc-probe in action.

Tokio might need some more cleanup or fixes according to the CI. I'll run through it again once the CI issues are fixed.

[melekes]

👍

[ancazamfir]

Looks good, I was able to get the tx info for IBC packets! Some small issue with the event key/ value encoding but orthogonal.
Thanks Thane!!

[ancazamfir]

Haven't looked at the code in a lot of detail but I was able to integrate with this in IBC so for me it's good to go 👍 Thanks Thane!!

[cargo-dep-bot]

This PR made the following dependency changes:

Added Packages (Duplicate versions in '()'):
	aead 0.3.2
	chacha20 0.6.0
	chacha20poly1305 0.7.1
	cipher 0.2.5
	crypto-mac 0.10.0
	eyre 0.6.3
	h2 0.2.7 (0.1.26)
	hkdf 0.10.0
	hmac 0.10.1
	indenter 0.3.0
	keccak 0.1.0
	merlin 2.0.0
	mio-named-pipes 0.1.7
	pin-project 0.4.27 (1.0.2)
	pin-project-internal 0.4.27 (1.0.2)
	pin-project-lite 0.1.11 (0.2.0)
	poly1305 0.6.1
	tendermint-p2p 0.17.0-rc3
	tokio-util 0.3.1
	tracing-futures 0.2.4
	universal-hash 0.4.0
	x25519-dalek 1.1.0

Removed Packages (Remaining versions in '()'):
	bytes 0.6.0 (0.5.6, 0.4.12)
	mio 0.7.6 (0.6.22)
	ntapi 0.3.6

Updated Packages:
	async-tungstenite: 0.10.0 -> 0.9.3
	http-body: 0.4.0 -> 0.3.1
	hyper: 0.14.0-dev -> 0.13.9
	tokio: 0.3.4 -> 0.2.23
	tokio-macros: 0.3.1 -> 0.2.6