sql: fix leak of prepared statements

[andreimatei]

The connExPrepStmtsAccessor.DeleteAll() method was not properly implemented and
"leaked" the memory accounting for prepared statements. This could lead
to crashes, like showed below.

I've looked at the various open issues around memory leak crashes, but
unfortunately I think it's unlikely this fixes those. Anyway, I'll
backport.

root@127.0.0.1:53405/defaultdb> begin; select 1;
root@127.0.0.1:53405/defaultdb OPEN> prepare x as select 1;
root@127.0.0.1:53405/defaultdb OPEN> deallocate all;
root@127.0.0.1:53405/defaultdb OPEN> commit;
root@127.0.0.1:53405/defaultdb> ^D
panic: session: unexpected 10240 leftover bytes

Release note (bug fix): Fix a memory leak around DEALLOCATE and DISCARD
statements that could result in crashes with the "unexpected
leftover bytes" message.

[cockroach-teamcity]

This change is 

[knz]

thank you!

[andreimatei]

So it turns out that this patch is actually broken. The patch resets one namespace to the empty namespace, but that's not something you can do. Given the current code, it's only correct to reset prepStmtsNamespace to prepStmtsNamespaceAtTxnRewindPos and vice-versa; it's not correct to reset one of those guys to some other namespace that comes from thin air cause then the reference tracking that we attempt to do gets broken. This is all undocumented and I should just be ashamed of myself for writing all this code around the namespaces the way I did.
The proper fix is #33423 which introduces reference counting for prepared statements. In conjunction with that patch, this one works properly, so I'll just fold this into #33423 (which already included this commit, but I'll squash them into one because it's now clear that they're not independent commits).

Reviewable status: complete! 0 of 0 LGTMs obtained