Support Read-Only Mode for DA Ledger API Server

[clasoncjDA]

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There are many operational reasons why I may want to leave my ledger up and running but prevent updates/writes to the underlying ledger. This is a feature request for a ledger admin to be able to enable read-only mode for the ledger API.

Migration from one DAML ledger to another is the primary use case driving this feature request. Both from one ledger type to another and to support moving contracts from a pre-GA ledger to a more mature version that supports upgrade in place. The desire is to place a ledger into a read-only state to support DAML contract level exports into another ledger instance.

Acceptance Criteria:

1. All command submissions that update the ledger fail
2. All read commands succeed

[mziolekda]

Read only state could be entered automatically, triggered by security events