Fixes cockroachdb#67551. Fixes cockroachdb#67552. Fixes cockroachdb#67553. Touches cockroachdb#67562. Bounded-staleness read orchestration consists of two phases - negotiation and execution. Negotiation determines the timestamp to run the query at in order to ensure that the read will not block on replication or on conflicting transactions. Execution then uses this timestamp to run the read request. This commit implements the bounded staleness server-side negotiation fast-path. This fast-path allows a bounded staleness read request that lands on a single range to perform its negotiation phase and execution phase in a single RPC. The server-side negotiation fast-path provides two benefits: 1. it avoids two network hops in the common-case where a bounded staleness read is targeting a single range. This in an important performance optimization for single-row point lookups. 2. it provides stronger guarantees around minimizing staleness during bounded staleness reads. Bounded staleness reads that hit the server-side fast-path use their target replica's most up-to-date resolved timestamp, so they are as fresh as possible. Bounded staleness reads that miss the fast-path and perform explicit negotiation (see below) consult a cache, so they may use an out-of-date, suboptimal resolved timestamp, as long as it is fresh enough to satisfy the staleness bound of the request. The commit then uses this new functionality to implement the `(*Txn).NegotiateAndSend` method detailed in the bounded staleness RFC. `NegotiateAndSend` is a specialized version of `Send` that is capable of orchestrating a bounded-staleness read through a transaction, given a read-only BatchRequest with a `min_timestamp_bound` set in its Header. If the method returns successfully, the transaction will have been given a fixed timestamp equal to the timestamp that the read-only request was evaluated at.

kv: introduce bounded staleness BatchRequest Header fields #67551

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