Use curlev_context_t error buffer for sxi_retry_* functions

Local context buffer should be used instead of global one. sxi_retry_init() must be now used after sxi_cbdata_create() calls.
Use sxi_cbdata_seterr() calls in functions that have access to curlev_context_t
Cosmetics

Do not leak cbdata on retry callback.

Fix memleak when user callback returns error for file upload.

Do not unref cbdata when cluster_query_ev fails. Polling should occur and call cbdata_finish function which eventually should free that pointers.

Fix error handling in compute_headers_url(): fix memleak when sxi_sha1_init() fails, set error messages while sha update functions fail, set rc to -1 to not override stored error messages.

Fix check for pointers: check for context reference in case of sxi_cbdata_is_tag fail, fix pointer checking in curlm_check().

Remove unnecessary vars.

Check for error returned from sxi_set_retry_cb(). Previously this function was not check if it returns an error.

Free memory taken by cbdata in case of sxi_cluster_query_ev_retry() fail in multi_upload routine.