Made sieve_match_context use a proper pool in stead of the datastack.

src/lib-sieve/ext-envelope.c

src/lib-sieve/plugins/body/tst-body.c

src/lib-sieve/plugins/enotify/tst-notify-method-capability.c

src/lib-sieve/plugins/environment/tst-environment.c

src/lib-sieve/plugins/imap4flags/tst-hasflag.c

src/lib-sieve/plugins/regex/mcht-regex.c

src/lib-sieve/plugins/relational/mcht-count.c

src/lib-sieve/plugins/variables/tst-string.c

src/lib-sieve/sieve-match.c

src/lib-sieve/sieve-match.h

src/lib-sieve/tst-address.c

src/lib-sieve/tst-header.c

src/testsuite/tst-test-error.c

src/testsuite/tst-test-result.c