schannel: reuse the send buffer

[hunterinvariants]

Reuse a connection-local buffer for encrypted Schannel writes instead of allocating and freeing a buffer for every send. Grow it when needed and release it when the connection is closed.

In a native Windows debug build, 30 sequential HTTPS transfers over one reused connection changed the send-buffer heap activity from 30 allocations and 30 frees to one allocation and one close-time free.

Verified with checksrc, MinGW 32-bit and 64-bit Schannel builds, and native Windows 11 Schannel runs.

[hunterinvariants]

One CI job was cancelled at the 15-minute limit: mingw, AM ucrt-x86_64 wolfssl c-ares U. This configuration does not exercise the Schannel code changed by this PR. The other 212 checks, including the Windows Schannel jobs, passed.

Could someone with workflow permissions rerun the cancelled job?

[jay]

Thanks