Write journal entry length prefix and payload in a single BufferedChannel write

[merlimat]

Motivation

On the journal append path, each entry is written to the BufferedChannel as two separate calls — the 4-byte length prefix and then the payload:

bc.write(lenBuff);
bc.write(qe.entry);

Each write() acquires the channel monitor, so every journal entry pays two lock acquisitions on a monitor that is also contended by the force-write thread (forceWrite() takes the same lock to reset unpersistedBytes), plus two rounds of position/unpersisted-bytes accounting.

Changes

• Add a BufferedChannel.write(ByteBuf src1, ByteBuf src2) overload that copies both buffers into the write buffer under a single lock acquisition, with a single position/unpersisted-bytes update. The two source buffers are passed as separate arguments — deliberately avoiding a CompositeByteBuf, which would add a per-entry allocation and component bookkeeping.
• The existing single-buffer write() is refactored onto the same internal helpers, with identical behavior.
• Journal now writes the length prefix and entry payload with one call.
• SlowBufferedChannel (test injection hook) overrides the new method so the configured add-delay still applies.
• New unit test testWriteTwoBuffers covers boundary crossings: payloads that fit, exactly fill, straddle the write-buffer capacity, and exceed it (multiple internal flushes in one call), verifying position() and the resulting file contents.

Verified: BufferedChannelTest (9/9, including the new test), BookieJournalTest, BookieWriteToJournalTest, BookieJournalForceTest, BookieJournalPageCacheFlushTest, BookieJournalNoSyncTest, BookieJournalMaxMemoryTest (30/30), and module checkstyle.