Closes#2812
This PR adds the flag --for-zero-height to gaiad export, which runs several alterations to the application state to prepare for restarting a new chain in a consistent fashion.
It also:
* Moves Gaia's export code to cmd/gaia/app/export.go for cleaner separation.
* Fixes an inconsistency where we treated the initChainer as happening at height -1 - it should now happen at height 0, since the first header sent by Tendermint has height 1.
* Runs the runtime invariant checks on start (in initChainer)
* Adds a few auxiliary functions to clear slashing periods
* Removes the Height field from Delegation objects in x/stake, which was not used anywhere
* Update docs/sdk/clients.md
* organize ADR directory like tendermint
* docs: move spec-proposals into spec/
* remove lotion, moved to website repo
* move getting-started to cosmos-hub, and voyager to website
* docs: move lite/ into clients/lite/
* move introduction/ content to website repo
* move resources/ content to website repo
* mv sdk/clients.md to clients/clients.md
* mv validators to cosmos-hub/validators
* move deprecated sdk/ content to _attic
* sdk/modules.md is duplicate with modules/README.md
* consolidate remianing sdk/ files into a single sdk.md
* move examples/ to docs/examples/
* mv docs/cosmos-hub to docs/gaia
* Add keys/accounts section to localnet docs
Christopher Goes
Alexander Bezobchuk
Alessio Treglia
gamarin2