Add log_statement_max_length GUC to limit logged statement text

Add log_statement_max_length GUC to limit logged statement text

Very large statements can make server logs grow unexpectedly. This is
particularly painful when applications accidentally or intentionally
send huge literal values and statement logging is enabled: the full
statement text may be written to the log even when DBA sees only
its leading part is useful for normal operations.

This commit adds log_statement_max_length GUC that limits the number
of bytes of statement text emitted by statement logging. The setting
applies to statements logged by log_statement, log_min_duration_statement,
log_min_duration_sample, and log_transaction_sample_rate. A positive
value truncates the logged statement body to at most that many bytes,
zero  logs an empty statement body, and the default value -1 preserves
the existing behavior of logging statements in full.

Truncation is byte-based, matching the GUC unit, but it clips only
at multibyte character boundaries so that the log output remains valid.
This setting does not affect statements logged because of
log_min_error_statement; handling error-statement logging can be
considered separately.

Author: Jim Jones <jim.jones@uni-muenster.de>
Author: Kirill Gavrilov <diphantxm@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Maxym Kharchenko <maxymkharchenko@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CA+E0NR4S+NC6+QHyY_vUuQZMzLhKqczMx-jJVqtjAxF6+=JwAA@mail.gmail.com