PARQUET-11: Reduce memory pressure when reading footers

This will intern the following strings columns:

SchemaElement.name;
KeyValue.key
KeyValue.value
 ColumnMetadata.(list<string>) path_in_schema
 ColumnChunk.file_path
 FileMetaData.created_by

All of these are likely to be highly repeated.

Heap analysis before applying this patch (some silly obfuscation applied):
```
Object header size: 12 bytes
Total size of all object headers: 214,501K (33.6%)

 #instances    (Average) object   Total header     Class name
                    size              size                   
-------------------------------------------------------------
   4,405,394         32         51,625K (8.1%)     String
   4,403,224         44         51,600K (8.1%)     char[]
   1,894,774         24         22,204K (3.5%)     long[]
   1,893,181         33         22,185K (3.5%)     Object[]
   1,892,932         24         22,182K (3.5%)     j.u.BitSet
   1,892,650         24         22,179K (3.5%)     j.u.ArrayList
     944,858         32         11,072K (1.7%)     parquet.format.ColumnChunk
     944,858         88         11,072K (1.7%)     parquet.format.ColumnMetaData
```

```
DUPLICATE STRING STATS

Total strings: 4,405,394  Unique strings: 6,837  Duplicate values: 1,139  Overhead: 328,790K (51.5%)

Top duplicate strings:
    Ovhd         Num char[]s   Num objs   Value

 42,831K (6.7%)   783213      783213      "user"
 12,628K (2.0%)   161647      161647      "user_extra"
  7,446K (1.2%)   105907      105907      "another_column"
  6,751K (1.1%)    86424       86424      "column_a"
  6,270K (1.0%)   100333      100333      "column_b"
  6,098K (1.0%)    97581       97581      "column_c"
  5,748K (0.9%)    91972       91972      "column_d"
  5,486K (0.9%)    58528       58528      "column_e"
  5,029K (0.8%)    58528       58528      "column_f"
..... and so on.
```

After applying this patch, the strings disappear from the profile (next we need to deal with duplicate arrays...):

```
Object header size: 12 bytes
Total size of all object headers: 230,459K (36.1%)

 #instances    (Average) object   Total header     Class name
                    size              size                   
-------------------------------------------------------------
   3,926,516         24         46,013K (7.2%)     long[]
   3,924,923         33         45,995K (7.2%)     Object[]
   3,924,674         24         45,992K (7.2%)     j.u.BitSet
   3,924,392         24         45,988K (7.2%)     j.u.ArrayList
   1,959,233         88         22,959K (3.6%)     parquet.format.ColumnMetaData
   1,959,233         32         22,959K (3.6%)     parquet.format.ColumnChunk

```

```
 DUPLICATE STRING STATS

Total strings: 7,004  Unique strings: 5,808  Duplicate values: 621  Overhead: 103K (0.0%)

Top duplicate strings:
    Ovhd         Num char[]s   Num objs   Value


===================================================

```

(that's right, they no longer show up!)