vttablet: fix deadlock in schema engine
vttablet: fix deadlock in schema engine

The new historian reuses the pool from schema engine. But that pool
has only one connection. I found a deadlock where se calls the
historian with that conn open, and the historian in turn requests
a connection for its own reload. This causes a deadlock. This is
fixed by increasing pool size to 2.

Signed-off-by: Sugu Sougoumarane <b9023a093a3fde2f5ed7cdf696263add90aef9db@gmail.com>

tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess