sql: implement SERIAL column type

Closes #4491.

There are two good options for this, each with tradeoffs. One is for SERIAL to
be an alias for DEFAULT unique_rowid and the other is to emulate Postgres's
SEQUENCEs.

Emulating SEQUENCE funnels every insert through a single point of contention,
but would be a drop-in replacement.

unique_rowid has no contention, is faster, and works for the common use of a
unique and increasing identifier (modulo overlapping transactions).
Unfortunately, it may also cause user confusion and break ORMs that (correctly
in Postgres) assume that SERIAL starts at 0 and (incorrectly) that SERIAL has no
holes. An interesting side benefit of unique_rowid is that it prevents the
problem of inadvertantly exposing user growth numbers (sign up for a service
once per day and keep track of the userids), which is a common issue with
SERIAL.

If it was known that user confusion and ORM compatibility would not be an issue,
unique_rowid would be the clear decision. Since it's still early enough to run
small experiments, this is the approach used here. If this results in sufficient
evidence that it was the wrong decision, a later version of Cockroach will
switch the behavior to emulate SEQUENCEs. Existing SERIAL columns will, however,
retain the old behavior.

sql/parser/col_types.go

@@ -63,6 +63,7 @@ var (
 	intColTypeInteger  = &IntColType{Name: "INTEGER"}
 	intColTypeSmallInt = &IntColType{Name: "SMALLINT"}
 	intColTypeBigInt   = &IntColType{Name: "BIGINT"}
+	intColTypeSerial   = &IntColType{Name: "SERIAL"}
 )
 
 func newIntBitType(n int) *IntColType {
@@ -86,6 +87,12 @@ func (node *IntColType) Format(buf *bytes.Buffer, f FmtFlags) {
 	}
 }
 
+// IsSerial returns true when this column should be given a DEFAULT of a unique,
+// incrementing function.
+func (node *IntColType) IsSerial() bool {
+	return node.Name == intColTypeSerial.Name
+}
+
 // Pre-allocated immutable float column types.
 var (
 	floatColTypeReal   = &FloatColType{Name: "REAL"}

sql/parser/parse_test.go

@@ -70,6 +70,7 @@ func TestParse(t *testing.T) {
 		{`CREATE TABLE a (b STRING)`},
 		{`CREATE TABLE a (b STRING(3))`},
 		{`CREATE TABLE a (b FLOAT)`},
+		{`CREATE TABLE a (b SERIAL)`},
 		{`CREATE TABLE a (b INT NULL)`},
 		{`CREATE TABLE a (b INT NOT NULL)`},
 		{`CREATE TABLE a (b INT PRIMARY KEY)`},