* Slight cleanup around mysql ha lock implementation
* Removes some duplication around lock table naming
* Escapes lock table name with backticks to handle weird characters
* Lock table defaults to regular table name + "_lock"
* Drop lock table after tests run
* Add `ha_enabled` option for mysql storage
It defaults to false, and we gate a few things like creating the lock
table and preparing lock related statements on it
* Add max_parallel parameter to MySQL backend.
This limits the number of concurrent connections, so that vault does not die
suddenly from "Too many connections".
This can happen when e.g. vault starts up, and tries to load all the
existing leases in parallel. At the time of writing this, the value
ExpirationRestoreWorkerCount in vault/helper/consts/const.go is set to
64, meaning that if there are enough leases in the vault's DB, it will
generate AT LEAST 64 concurrent connections to MySQL when loading the
data during start-up. On certain configurations, e.g. smaller AWS
RDS/Aurora instances, this will cause Vault to fail startup.
* Fix a typo in mysql storage readme
