sql: add new builtins to save and restore a session

[rafiss]

To allow SQL pods to scale down and back up, without user impact, we should add two new builtins:

1. one that “saves” (or “pickles”) the session state
2. and another that “restores” (or “unpickles”) the session state.

This state can just be returned directly to the caller (in this case sqlproxy) in some encoded form. sqlproxy is in charge of managing this encoded state.

The initial implementation of these builtins only needs to include the values of all session variables in the state.

Behavior:

• Sessions will not be suspended when there is an open transaction. Therefore, the "save" builtin should return an error if it's called in the middle of a transaction.
• Consider that these builtins will be running inside of the user's session. We could add something to make sure the user matches user in the saved session state, but I don't think that's necessary. (Essentially, this first version of the command is a shortcut for SHOWing each session var, and then SETing them all later.)

Out of scope for this issue:

• Including prepared statements in the session state. (We possibly may not even need this ever, since I believe drivers auto re-prepare statements, but more investigation needed.)
• Temporary tables

Epic CRDB-1464

[otan]

Rudimentary research:

• how do we feel about sequences:
  

  cockroach/pkg/sql/sessiondata/session_data.go

  Lines 45 to 47 in d33af93

  	// SequenceState gives access to the SQL sequences that have been
  	// manipulated by the session.
  	SequenceState *SequenceState

• I'm guessing we copy "LocalOnlySessionData":

  cockroach/pkg/sql/sessiondata/session_data.go

  Line 130 in d33af93

  type LocalOnlySessionData struct {

  , in which case we'll need to protobuf all of that.

[otan]

ah, sequences are already serialisable.

[otan]

What guarantees do we have about migrating settings between different version of CockroachDB?

[otan]

ah wait, there's this thing for sequences:

cockroach/pkg/sql/sessiondata/session_data.go

Lines 268 to 270 in 50ef2fc

	// SequenceCache stores sequence values which have been cached using the
	// CACHE sequence option.
	SequenceCache SequenceCache