Fix float value in create statement

[JaySon-Huang]

If we create a table with float devalue value

create table tt(i Int32) engine = Null
alter table tt add column f32 Float32 default 1.234

We get a schema like this: db/metadata/default/tt.sql

ATTACH TABLE tt
(
    i Int32,
    f32 Float32 DEFAULT CAST(Decimal32_1.234 AS Float32)
)
ENGINE = Null

If we stop and start TiFlash again, we get exception when loading table schema from disk

2019.08.23 22:32:12.919765 [ 1 ] <Error> Application: DB::Exception: Cannot create table from metadata file /Users/jayson/Projects
/pingcap/algernon/log/tiflash0/data/db/metadata/default//tt.sql, error: DB::Exception: Unknown identifier: Decimal32_1, stack trac
e:
0. 0   theflash                            0x00000001083d9b1e _ZN10StackTraceC2Ev + 30
1. 1   theflash                            0x00000001083d9b55 _ZN10StackTraceC1Ev + 21
2. 2   theflash                            0x00000001085a1903 _ZN2DB9ExceptionC2ERKNSt3__112basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEEi + 83
3. 3   theflash                            0x000000010859e3a3 _ZN2DB9ExceptionC1ERKNSt3__112basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEEi + 35
4. 4   theflash                            0x000000010fb5f67e _ZN2DB18ExpressionAnalyzer18collectUsedColumnsEv + 3646
5. 5   theflash                            0x000000010fb5b79e _ZN2DB18ExpressionAnalyzerC2ERKNSt3__110shared_ptrINS_4IASTEEERKNS_7ContextERKNS2_INS_8IStorageEEERKNS_17NamesAndTypesListERKNS1_6vectorINS1_12basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEENSL_ISN_EEEEmbRKNS1_13unordered_mapISN_NS_14SubqueryForSetENS1_4hashISN_EENS1_8equal_toISN_EENSL_INS1_4pairIKSN_ST_EEEEEE + 2094
6. 6   theflash                            0x000000010fb603b0 _ZN2DB18ExpressionAnalyzerC1ERKNSt3__110shared_ptrINS_4IASTEEERKNS_7ContextERKNS2_INS_8IStorageEEERKNS_17NamesAndTypesListERKNS1_6vectorINS1_12basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEENSL_ISN_EEEEmbRKNS1_13unordered_mapISN_NS_14SubqueryForSetENS1_4hashISN_EENS1_8equal_toISN_EENSL_INS1_4pairIKSN_ST_EEEEEE + 128
7. 7   theflash                            0x000000010fc331e0 _ZN2DBL12parseColumnsERKNS_17ASTExpressionListERKNS_7ContextE + 2304

This PR make decimal value wrote as a string in CAST statement.

ATTACH TABLE tt
(
    i Int32,
    f32 Float32 DEFAULT CAST('1.234' AS Float32)
)
ENGINE = Null

[JaySon-Huang]

/run-integration-tests

[hanfei1991]

LGTM

[JaySon-Huang]

/run-integration-tests