MFV r328225: 8603 rename zilog's "zl_writer_lock" to "zl_issuer_lock"

illumos/illumos-gate@cf07d3da9915c0d22da8f59e991639f819463cef

https://www.illumos.org/issues/8603:

To help make the ZIL's code more understandable, it was suggested that
the zilog_t's "zl_writer_lock" field should be renamed to "zl_issuer_lock".

Reviewed by: C Fraire <cfraire@me.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Prakash Surya <prakash.surya@delphix.com>