record statement boundaries

Attached is a patch from Joachim Wieland that changes the parser to
optionally maintain a list of the source texts of the statements in the
input query string. Currently debug_query_string is used for that
purpose -- this is incorrect when multiple comma-separated statements
are concatenated together in a single query string.

The patch maintains the list by having the parser record when each
statement ends; the lexer's current position is used to determine the
source text that corresponds to the current statement. A new global
variable "current_statement" has been introduced to replace
"debug_query_string" (which has been removed).

Two issues/questions:

(1) Joachim's patch removes leading and trailing whitespace, and any
trailing semi-colon from the statement's text. Is this the right
behavior? I'm not sure, I can see an argument for both sides.

(2) Some of the logging code (e.g. log_after_parse(), the code for
log_min_duration_statement, etc.) needs to be updated to use
current_statement rather than the user-submitted query string. I'll do
that shortly.

Patch from Joachim Wieland, reviewed by Neil Conway.

-Neil