kvserver: RPC to lock a key span for exclusive access

[erikgrinaker]

As described in #69380, even though we want to move to MVCC-compliant operations (#70429), we will still have some use for non-MVCC operations such as RevertRange, e.g. during streaming replication cutover. However, we should formalize the use of these operations by requiring the key span to be globally locked for access, to ensure that no other components are interacting with the span during this time. This would likely involve some sort of LockSpan RPC used to acquire and release global locks for a key span, with a lock token that must be used by when sending non-MVCC RPC commands like RevertRange, ClearRange, and AddSSTable with historical timestamps.

Jira issue: CRDB-10073

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