cli: provide auto_trace as replacement to SHOW TRACE FOR <stmt>

[knz]

The SHOW TRACE FOR syntax form was removed from CockroachDB SQL
because it was hard to implement correctly server-side.

However this does not mean that the need for it did not exist. Folk
were moderately unhappy to lose a feature that was exploited
productively during performance analysis and general troubleshooting.

To respond to the need, this patch instead provides a feature in
cockroach sql that replaces the previous SHOW TRACE FOR
syntax. For example:

> \set auto_trace on
> SELECT 1
+----------+
| ?column? |
+----------+
|        1 |
+----------+
(1 row)

Time: 327.38µs

+----------------------------------+------------+------------------------------------------------------------------+---------------------------------------+----------+-------------------+------+
|            timestamp             |    age     |                             message                              |                  tag                  | location |     operation     | span |
+----------------------------------+------------+------------------------------------------------------------------+---------------------------------------+----------+-------------------+------+
| 2018-07-20 15:25:29.09039+00:00  | 0s         | === SPAN START: session recording ===                            |                                       |          | session recording |    0 |
| 2018-07-20 15:25:29.09041+00:00  | 19µs997ns  | [NoTxn pos:22] executing Sync                                    | [n1,client=127.0.0.1:16847,user=root] |          | session recording |    0 |
| 2018-07-20 15:25:29.09048+00:00  | 89µs917ns  | [NoTxn pos:23] executing ExecStmt: SELECT 1                      | [n1,client=127.0.0.1:16847,user=root] |          | session recording |    0 |
[...]
+----------------------------------+------------+------------------------------------------------------------------+---------------------------------------+----------+-------------------+------+
(22 rows)

Time: 1.836533ms

That is, for every statement executed, the shell also produces the
trace for that statement in a separate result below.

A trace is also produced in case the statement produces a SQL error.

It works as follows:

• a new client-side option auto_trace is provided (configurable with
  \set).
• when set to non-off, it causes every subsequent statement to be
  enclosed in:
  • SET tracing = off; SET tracing = on, <options...> at the
    beginning where <options...> is the content of the variable
    auto_trace.
  • SET tracing = off; SHOW <option> TRACE FOR SESSION at the end,
    where <option> is either empty or kv if auto_trace contains
    kv.

Release note (cli change): A new client-side option auto_trace is
provided to facilitate usage of session tracing.

[cockroach-teamcity]

This change is 

[knz]

cc @tschottdorf you may have an opinion on the usability

[jordanlewis]

LGTM. This makes me wonder whether we should also add a non-"auto" version of this, e.g. \trace ANY SQL GOES HERE

[knz]

@jordanlewis that was my first idea but the architecture of our CLI does not allow this without a major rewrite. So no (not yet).

[knz]

TFYR

bors r+

[craig]

Build succeeded

• GitHub CI (Cockroach)

[tbg]

I like this, thanks Raphael!

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-- Tobias