Configurable BAL retention policy

[mirgee]

Adds new CLI options:

• --Xchain-pruning-bals-retained - sets the number of retained BALs, must be <= --Xchain-pruning-blocks-retained
• --Xchain-pruning-bals-frequency - the time in blocks after which BAL pruning is executed
• --Xchain-bal-pruning-enabled - whether BAL pruning is enabled (default: true)

The relationship between BALs and blocks retained is currently enforced because both pruning task use forkBlocks, but only one can be responsible for deleting them (which is the block pruning task) without further changes. However, this restriction only applies if --Xchain-pruning-enabled, which it isn't by default, so removing this restriction was considered lower priority for now.

Breaking changes:

The option --Xchain-pruning-blocks-blocks-limit was renamed to --Xchain-pruning-blocks-bals-limit, as it sets the lower bound on the number of BALs retained (and then only indirectly on blocks retained). This is debatable as if BAL pruning is disabled, it still bounds blocks retained.

By default, the minimal number of BALs retained (and hence the minimum number of blocks retained) is 3533 * 32 = 417792. This lower bound can be overriden by setting a lower --Xchain-pruning-blocks-bals-limit, for example for purposes of networks without BALs or WSP.

TODO:

• Pruning blocks but not BALs is violating the invariant BALs retained <= blocks retained.
• Test.

Based on #9629 .