pick #40367
Meet code dump when using single stream multi table load:
```
SUMMARY: AddressSanitizer: heap-buffer-overflow /root/doris/be/src/io/fs/multi_table_pipe.cpp:99:22 in doris::io::MultiTablePipe::dispatch(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&, char const*, unsigned long, doris::Status (doris::io::KafkaConsumerPipe::*)(char const*, unsigned long))
```
1. It is hard to guaranteed that msg is a C-style string ending in '\0'
character. If not, it may cause the core dump to access memory out of
bounds.
2. It is not need to calculate the length of msg twice.
Therefore, deleting the logic that using strlen to calculate the length
of msg.
In previous, when using file scan node(eq, querying hive table), the max number of scanner for each scan node
will be the `doris_scanner_thread_pool_thread_num`(default is 48).
And if the query parallelism is N, the total number of scanner would be 48 * N, which is too many.
In this PR, I change the logic, the max number of scanner for each scan node
will be the `doris_scanner_thread_pool_thread_num / query parallelism`. So that the total number of scanners
will be up to `doris_scanner_thread_pool_thread_num`.
Reduce the number of scanner can significantly reduce the memory usage of query.
For routine load (kafka load), user can produce all data for different
table into single topic and doris will dispatch them into corresponding
table.
Signed-off-by: freemandealer <freeman.zhang1992@gmail.com>
