MEDIUM: log: add compiling logic to logformat expressions

MEDIUM: log: add compiling logic to logformat expressions

split parse_logformat_string() into two functions:

parse_logformat_string() sticks to the same behavior, but now becomes an
helper for lf_expr_compile() which uses explicit arguments so that it
becomes possible to use lf_expr_compile() without a proxy, but also
compile an expression which was previously prepared for compiling (set
string and config hints within the logformat expression to avoid manually
storing string and config context if the compiling step happens later).

lf_expr_dup() may be used to duplicate an expression before it is
compiled, lf_expr_xfer() now makes sure that the input logformat is
already compiled.

This is some prerequisite works for log-profiles implementation, no
functional change should be expected.