The Dirk accountmanager was using a local scatter/gather concurrency
method to obtain wallets, however this uses the parallelism of the Vouch
server rather than the Dirk server. This chnages the Dirk
accountmanager to use a configuration value to select the concurrency
level.
This also standardizes the use of process concurency to allow for
hierarchical definition of the value.
This reintroduces prometheus metrics for the account manager module.
The metrics track the validating state of each account, and are found
under the `vouch_accountmanager_accounts_total` metric. The metrics are
differentiated using the `state` label.