… scoped leases

This is a very WIP implementation of transaction scoped locks.
The interface is garbage and not very considered.

Nevertheless it seems to mostly work. The biggest problem I see so far is that
at high concurrency there are cases where a lock can lay down all of the
relevant intents and then still fail to commit. This happens because the TS
cache information is lost due to a lease transfer before the txn record is layed
down. Such a possibility is a big problem for the lease use cases which assume
that an intent will be valid until the transaction expires. To some extent this
problem makes sense given the transaction expiration isn't well defined before
the txn record has been written.

Release note: None

- Txn.ForceHeartbeat() writes a transaction record.
- Txn.ExpiryTimestamp() returns the current expiry timestamp.
- TODOs for plumbing a special Intent that updates the ts cache when removed due to abort.

Release note: None

More plumbing for special intent that updates ts cache.
Remaining work is to somehow update the ts cache in cmd_resolve_intent.go
and cmd_resolve_intent_range.go,

Release note: None

Plumbing to update the ts cache when removing a "leasing intent".

Release note: None

Added proof sketch for lease/lock implementation.
Added description of alternative that does not have contention
between concurrent acquisitions of shared lease.

Release note: None